\makeatletter
\@ifundefined{HCode}
{\documentclass[screen, CRGEOS, Unicode, Thematic,published]{cedram}
%\usepackage[sort&compress,square,comma,authoryear]{natbib}
\usepackage[square,comma,authoryear]{natbib}
\usepackage{wasysym}
\usepackage[T1]{fontenc}
\newenvironment{Table}{\begin{table}}{\end{table}}
\renewcommand\bibsection{\section*{\refname}}
\newenvironment{noXML}{}{}
\def\tabnote#1{\vskip4pt\parbox{.98\linewidth}{#1}}
\def\sfrac#1#2{{#1}/{#2}}
\def\sofrac#1#2{(#1/#2)}
\def\botline{\\\hline}
\def\thead{\noalign{\relax}\hline}
\def\endthead{\noalign{\relax}\hline}
\def\tbody{\noalign{\relax}\hline}
\def\tsup#1{$^{{#1}}$}
\def\tsub#1{$_{{#1}}$}
\def\ndash{\text{--}}
\def\Lbreak{\newline}
\def\hbreak{{-\break}}
\def\ubreak{\break}
\RequirePackage{etoolbox}
\usepackage{upgreek}
\def\jobid{crgeos20220873}
%\graphicspath{{/tmp/\jobid_figs/web/}}
\graphicspath{{./figures/}}
\def\0{\phantom{0}}
\usepackage{soul}
\newcounter{runlevel}
\usepackage{multirow} 
\def\nrow#1{\@tempcnta #1\relax%
\advance\@tempcnta by 1\relax%
\xdef\lenrow{\the\@tempcnta}}
\def\morerows#1#2{\nrow{#1}\multirow{\lenrow}{*}{#2}}
\def\xmorerows#1#2{#2}
\let\MakeYrStrItalic\relax
\def\refinput#1{}
\def\back#1{}
\csdef{Seqnsplit}{\\}
\DOI{10.5802/crgeos.239}
\datereceived{2022-09-06}
\daterevised{2023-07-21}
\dateaccepted{2023-09-26}
\ItHasTeXPublished
\makeatletter
\g@addto@macro{\UrlBreaks}{\UrlOrds}
\gappto{\UrlBreaks}{\UrlOrds}
}
{
\PassOptionsToPackage{authoryear}{natbib}
\documentclass[crgeos]{article}
\usepackage[T1]{fontenc} 
\def\CDRdoi{10.5802/crgeos.239}
\makeatletter
\def\sfrac#1#2{{#1}/{#2}}
\let\hbreak\relax
\def\sofrac#1#2{(#1/#2)}
\let\newline\relax
\def\href#1#2{\url[#1]{#2}}
\def\xmorerows#1#2{\morerows{#1}{#2}}
\def\CDRsupplementaryTwotypes#1#2{}
\let\ubreak\relax
}
\makeatother

\usepackage{moreverb}

\dateposted{2023-11-22}
\datepublished{2024-02-23}
\begin{document}

\begin{noXML}

%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{Geo-hydrological Data \& Models}
%%\def\cdr@specialtitle@french{G\'eo-hydrologie, donn\'ees \& mod\`eles}
%\makeatother

\title{Hard rock aquifer architecture and water circulation levels in
the Strengbach critical zone observatory (France)}

\alttitle{Architecture des aquif\`{e}res rocheux et niveaux de
circulation d'eau dans l'observatoire de la zone critique du Strengbach
(France)}

%Chabaux Fran\c{c}ois
\author{\firstname{Fran\c{c}ois} \lastname{Chabaux}\IsCorresp}
\address{Institut Terre et Environnement de Strasbourg, Universit\'{e}
de Strasbourg, CNRS, ENGEES, 67000 Strasbourg, France}
\email[F. Chabaux]{fchabaux@unistra.fr}

%N\'{e}grel Philippe
\author{\firstname{Philippe} \lastname{N\'{e}grel}\IsCorresp}
\address{BRGM, 45060 Orl\'{e}ans, France}
\email[P. N\'{e}grel]{p.negrel@brgm.fr}

%Gal Frederick
\author{\firstname{Frederick} \lastname{Gal}\IsCorresp}
\addressSameAs{2}{BRGM, 45060 Orl\'{e}ans, France}
\email[F. Gal]{f.gal@brgm.fr}

%Viville Daniel
\author{\firstname{Daniel} \lastname{Viville}}
\addressSameAs{1}{Institut Terre et Environnement de Strasbourg,
Universit\'{e} de Strasbourg, CNRS, ENGEES, 67000 Strasbourg, France}

%Ranchoux Coralie
\author{\firstname{Coralie} \lastname{Ranchoux}}
\addressSameAs{1}{Institut Terre et Environnement de Strasbourg,
Universit\'{e} de Strasbourg, CNRS, ENGEES, 67000 Strasbourg, France}

%van der Woerd J\'{e}rome
\author{\firstname{J\'{e}rome} \lastname{Van der Woerd}}
\addressSameAs{1}{Institut Terre et Environnement de Strasbourg,
Universit\'{e} de Strasbourg, CNRS, ENGEES, 67000 Strasbourg, France}

%Delay Frederick
\author{\firstname{Frederick} \lastname{Delay}}
\addressSameAs{1}{Institut Terre et Environnement de Strasbourg,
Universit\'{e} de Strasbourg, CNRS, ENGEES, 67000 Strasbourg, France}

%Ackerer Julien
\author{\firstname{Julien} \lastname{Ackerer}}
\addressSameAs{1}{Institut Terre et Environnement de Strasbourg,
Universit\'{e} de Strasbourg, CNRS, ENGEES, 67000 Strasbourg, France}

%Aquilina Luc
\author{\firstname{Luc} \lastname{Aquilina}}
\address{G\'{e}osciences Rennes, Universit\'{e} Rennes, CNRS, 35000
Rennes, France}

%Diraison Marc
\author{\firstname{Marc} \lastname{Diraison}}
\addressSameAs{1}{Institut Terre et Environnement de Strasbourg,
Universit\'{e} de Strasbourg, CNRS, ENGEES, 67000 Strasbourg, France}

%Masson Fr\'{e}d\'{e}ric
\author{\firstname{Fr\'{e}d\'{e}ric} \lastname{Masson}}
\addressSameAs{1}{Institut Terre et Environnement de Strasbourg,
Universit\'{e} de Strasbourg, CNRS, ENGEES, 67000 Strasbourg, France}

%M\'{e}riaux Anne-Sophie
\author{\firstname{Anne-Sophie} \lastname{M\'{e}riaux}}
\addressSameAs{1}{Institut Terre et Environnement de Strasbourg,
Universit\'{e} de Strasbourg, CNRS, ENGEES, 67000 Strasbourg, France}

%Labasque Thierry   
\author{\firstname{Thierry} \lastname{Labasque}}
\addressSameAs{3}{G\'{e}osciences Rennes, Universit\'{e} Rennes, CNRS,
35000 Rennes, France}

%Lucas Yann
\author{\firstname{Yann} \lastname{Lucas}}
\addressSameAs{1}{Institut Terre et Environnement de Strasbourg,
Universit\'{e} de Strasbourg, CNRS, ENGEES, 67000 Strasbourg, France}

%Pezard Philippe Adrien
\author{\firstname{Philippe Adrien} \lastname{Pezard}}
\address{G\'{e}osciences Montpellier, CNRS, Universit\'{e} de
Montpellier, Place Eug\`{e}ne Bataillon, 34000 Montpellier, France}

%Reuschl\'{e} Thierry
\author{\firstname{Thierry} \lastname{Reuschl\'{e}}}
\addressSameAs{1}{Institut Terre et Environnement de Strasbourg,
Universit\'{e} de Strasbourg, CNRS, ENGEES, 67000 Strasbourg, France}

%Schmitt Anne-D\'{e}sir\'{e}e
\author{\firstname{Anne-D\'{e}sir\'{e}e} \lastname{Schmitt}}
\addressSameAs{1}{Institut Terre et Environnement de Strasbourg,
Universit\'{e} de Strasbourg, CNRS, ENGEES, 67000 Strasbourg, France}

%Stille Peter
\author{\firstname{Peter} \lastname{Stille}}
\addressSameAs{1}{Institut Terre et Environnement de Strasbourg,
Universit\'{e} de Strasbourg, CNRS, ENGEES, 67000 Strasbourg, France}

\author{\firstname{Robert} \lastname{Wyns}}
\addressSameAs{2}{BRGM, 45060 Orl\'{e}ans, France}

%Lerouge Catherine
\author{\firstname{Catherine} \lastname{Lerouge}\IsCorresp}
\addressSameAs{2}{BRGM, 45060 Orl\'{e}ans, France}
\email[C. Lerouge]{c.lerouge@brgm.fr}

%Dezayes Chrystel
\author{\firstname{Chrystel} \lastname{Dezayes}\IsCorresp}
\addressSameAs{2}{BRGM, 45060 Orl\'{e}ans, France}
\email[C. Dezayes]{c.dezayes@brgm.fr}

\thanks{French ANR program (Grant agreement no. ANR-15-CE06-0014),
REALISE program (CPER Etat Alsace 2003--2014), INSU-CNRS and French
Equipex program CRITEX}

\shortrunauthors

\begin{abstract}
We present an integrated petrological, petrophysical, and
hydrogeological study of the critical zone (CZ) developed in the
Hercynian granitic basement of the Strengbach watershed (Vosges Massif,
France) to characterize its deep architecture and water circulation
levels. For this purpose, six boreholes (50--120 m depth), from which
three are cored, and three piezometers (10--15 m depth) were drilled to
define the vertical extension and lateral variability of the main CZ
horizons. 

The Strengbach watershed is composed of a topsoil horizon of limited
vertical extension (0.8--1.2 m), a mobile saprolite level, and an
in-place fractured bedrock. The latter is subdivided into a few meters
thick saprock horizon, defined by open sub-horizontal fractures and a
deeper fractured bedrock horizon with steeply dipping fractures
(${>}$50\textdegree). In the north-facing slope, the vertical
extension of the mobile saprolite horizon increases from 
${\approx}$1--2 m at the top of the slope to ${\approx}$9 m downstream, close to
the valley bottom. In contrast, the south-facing and more easterly
slope shows a mobile saprolite horizon with limited vertical extension
(${\approx}$2--3 m thick). Such a difference is associated with the
existence of a knickpoint in the river bed, separating a downstream
zone marked by currently active erosion from an upstream one, less
prone to erosion, with preserved reliefs formed around 20~ka ago. 

The water circulation scheme within the Strengbach watershed involves
two different systems: a subsurface circulation within the shallow
aquifer, corresponding to the mobile saprolite horizon and the saprock,
and a deeper circulation in the fractured bedrock. The water
circulation in the fractured bedrock is controlled by fractures of
regional orientations, linked to the Vosges massif and the Rhine Graben
Tertiary tectonics, and partly to reactivated Hercynian fracture zones.
The unaltered bedrock was not reached by any of the three cores. These
results from the Strengbach CZ demonstrate the importance of
integrating geological history of the watershed, either the long-term
geological bedrock evolution or the Quaternary erosion patterns, to
better understand and model the CZ hydrological functioning at the
watershed scale.  
\end{abstract}

\begin{altabstract}
Nous pr\'{e}sentons ici une \'{e}tude p\'{e}trologique,
p\'{e}trophysique et hydrog\'{e}ologique int\'{e}gr\'{e}e de la zone
critique (ZC) d\'{e}velopp\'{e}e dans le socle granitique hercynien du
bassin versant du Strengbach (Massif des Vosges, France), afin de
caract\'{e}riser son architecture profonde et ses niveaux de
circulation d'eau. Dans ce but, six forages (50--120 m de profondeur),
dont trois carott\'{e}s, et trois pi\'{e}zom\`{e}tres (10--15 m de
profondeur) ont \'{e}t\'{e} r\'{e}alis\'{e}s, afin de d\'{e}finir
l'extension verticale et la variabilit\'{e} lat\'{e}rale des principaux
horizons de la ZC. 

Le bassin versant du Strengbach est compos\'{e} d'un horizon de sol
d'extension verticale limit\'{e}e (0,8--1,2 m), d'un niveau de
saprolite mobile, et d'une roche m\`{e}re en place fractur\'{e}e. Cette
derni\`{e}re se subdivise en un horizon de \guillemotleft{}~saprock~\guillemotright{} de quelques
m\`{e}tres d'\'{e}paisseur, d\'{e}fini par des fractures ouvertes
subhorizontales, et un horizon plus profond de roche m\`{e}re
fractur\'{e}e par des fractures \`{a} fort pendage
(${>}$50~\textdegree). Dans le versant orient\'{e} au nord, l'extension
verticale de l'horizon de saprolite mobile passe de ${\approx}$ 1--2 m 
au sommet du versant \`{a} ${\approx}$ 9 m en aval, au
voisinage du vallon. En revanche, le versant orient\'{e} au sud et plus
\`{a} l'est pr\'{e}sente un horizon de saprolite mobile d'extension
verticale limit\'{e}e (${\approx}$2--3 m d'\'{e}paisseur). Une telle
diff\'{e}rence est probablement li\'{e}e \`{a} l'existence d'un point
d'inflexion dans le lit de la rivi\`{e}re (\guillemotleft{}~Knickpoint~\guillemotright{}), s\'{e}parant
une zone aval marqu\'{e}e par une \'{e}rosion actuellement active d'une
zone amont, moins sujette \`{a} l'\'{e}rosion, comprenant des reliefs
pr\'{e}serv\'{e}s form\'{e}s il y a environ 20 ka. 

Le sch\'{e}ma de circulation d'eau dans le bassin versant du Strengbach
implique deux syst\`{e}mes diff\'{e}rents de circulation: une
circulation dans l'aquif\`{e}re de surface, constitu\'{e} de
l'horizon  de saprolite mobile et du niveau de \guillemotleft{}~saprock~\guillemotright{}, et une
circulation plus profonde dans la roche m\`{e}re fractur\'{e}e. La
circulation d'eau dans la roche m\`{e}re fractur\'{e}e est
contr\^{o}l\'{e}e par des fractures d'orientations r\'{e}gionales,
li\'{e}es \`{a} la tectonique tertiaire du massif des Vosges et du
foss\'{e} rh\'{e}nan, ayant r\'{e}activ\'{e} en partie au moins des
fractures hercyniennes. Aucune des trois carottes n'a atteint le
substrat rocheux non alt\'{e}r\'{e}. Ces r\'{e}sultats obtenus sur le
bassin versant du Strengbach d\'{e}montrent l'importance
d'int\'{e}grer l'histoire g\'{e}ologique du bassin versant, qu'il
s'agisse de l'\'{e}volution g\'{e}ologique long-terme de son substratum
ou des processus d'\'{e}rosion Quaternaire qui l'ont affect\'{e},
pour mieux comprendre et mod\'{e}liser le fonctionnement hydrologique
de la ZC \`{a} l'\'{e}chelle du bassin versant.
\end{altabstract}

\keywords{\kwd{Deep critical zone architecture} 
\kwd{Hard rock aquifer} 
\kwd{Water circulation levels} 
\kwd{Geological heritage} 
\kwd{Strengbach Critical Zone Observatory}}

\altkeywords{\kwd{Architecture profonde de la zone critique}
\kwd{Aquif\`{e}re rocheux}
\kwd{Niveaux de circulation de l'eau}
\kwd{H\'{e}ritage g\'{e}ologique}
\kwd{Observatoire de la Zone Critique du Strengbach}}

\maketitle

\vspace*{-3pt}

\begin{quote}
\textit{``Hydrogeology is a discipline in which
everything happens, by definition, hidden from view in the crevices of
the underground environment. What is required of the hydrogeologist is
to explain what happens underground\,\ldots''}~\citep{deMarsily2004}.
\end{quote}

\vspace*{9pt}

\twocolumngrid

\end{noXML}

\section{Introduction}

The Critical Zone (CZ) is classically defined as ``the near-surface
layer where water, rock, air, and life meet in a dynamic interplay that
sculpts landscapes, generates soils, and builds foundation for Earth's
terrestrial ecosystems'' (National Research Council NRC 2001).
Its vertical extension, beyond the few
meters classically studied through the analysis of weathering profiles,
is however still an open question \citep[e.g.,][]{Riebeetal2017}. This
lack of knowledge is probably associated with difficulties in sampling
and gathering information from the deep CZ shielded by several meters
of weathered materials \citep{Riebeetal2017,Holbrooketal2019}. A
commonly accepted view---but still debated 
\citep[see][]{Riebeetal2017}---is to assimilate the CZ lower boundary to
the level where subsurface materials grades into compact rocks no
longer affected by meteoric fluids \citep{Riebeetal2017}.
Thus, defining the deep CZ
means characterizing the zones of groundwater flow connected with
surface circulation and distinguishing them from deeper levels without
evidence of water circulation or with circulations unrelated to surface
processes. Parallel to the rise of the CZ concept, hydrogeological
studies on continental water resources led to the definition of a
conceptual structural scheme for Hard Rock Aquifers (HRA), i.e.,
aquifers in granitic and silicate metamorphic bedrocks 
\citep[review in][]{Lachassagneetal2021}. The theoretical HRA scheme is a
stratiform structure beneath ground level and above non-weathered
rocks, consisting of an unconsolidated saprolite (or regolith) level (a
clay-rich zone formed by the weathering of silicate rocks)~superimposed
on a permeable fissured layer. The latter is characterized by a
secondary fracture network with fracture density decreasing over depth
until reaching the unweathered bedrock 
\citep[e.g.,][]{Dewandeletal2011,Lachassagneetal2021}. In this type of
structure, the hydraulic conductivity of the fractured level is\break mainly
controlled by the permeability of sub-horizontal fractures, which is
higher than that of other fracture families, and which also goes with
the usual development of sub-horizontal groundwater flow at the
regional scale. For its part, the existence of a secondary fracture
network is interpreted as a consequence of alteration processes 
\citep[e.g.,][]{Lachassagneetal2021, Worthingtonetal2016}.
The HRA scheme has
been built on the basis of a large context of field observations issued
from weathering profiles \citep[review in][]{Lachassagneetal2021}. It
typically concerns the first tens of meters beneath ground level
\citep{Wynsetal1999}
and is undoubtedly a relevant model of CZ
architecture in silicate rock context. However, the HRA model, similar
to the CZ concept, is not very detailed regarding the exact location of
their lower bound, i.e., the transition between the fissured horizon
housing groundwater flow and the fractured bedrock underneath.
Characterizing this boundary involves comparing, at the same location,
the horizons where flow occurs and the structural and petrological data
of the host rocks. This idea motivated in recent years diverse works to
equip observatory sites of the CZ with deep boreholes of several tens
of meters. Those should increase the cross-knowledge between the
structure and architecture of the CZ over depth and the nature and
types of water circulating within the CZ 
\citep[e.g.,][]{Ayraudetal2008,Dewandeletal2011,Dewandeletal2017,
Brantleyetal2013,Bussetal2013,Orlandoetal2016,Chabauxetal2017,Riebeetal2017,
Hahmetal2019,Holbrooketal2019,Ackereretal2021}. Such cross-information
has also proved to be important for calibrating relatively recently
developed hydrogeophysical investigation methods to better understand
the structure and hydrological functioning of the CZ 
\citep[e.g.,][]{GrelleGuadagno2009,  Sailhacetal2009,
Befusetal2011,Johnsonetal2012,
Massonetal2012,Holbrooketal2014,Parsekianetal2015,Orlandoetal2016,
Lesparreetal2020,Chaffautetal2020}.

{The Strengbach Critical Zone Observatory (CZO) is a fully equipped
environmental observatory with meteorological, hydrological, and
geochemical data recorded since 1986} (\url{https://ohge.unistra.fr}).
It is one of the reference sites of the French OZCAR network (Critical
Zone Observatories: Research and 
Application---\url{https://www.ozcar-ri.org}). Specifically, the Strengbach
observatory partly targets the study of weathering processes on the
basis of stream water, spring water, and soil analysis
\citep[][and references therein]{Vivilleetal2012,Vivilleetal2017,
Pierretetal2018,Chabauxetal2019}. It
has been equipped with six 50--100 m-deep boreholes and three shallower
10--15 m-deep boreholes (hereinafter referred to as piezometers) in
order to better understand the structure and circulation of water in
the bedrock of the watershed. In the present study, we propose a
synthesis of the main structural, petrological, and petrophysical data
collected from the three deep-cored boreholes and the hydro-geophysical
investigations acquired through the boreholes. These data, combined
with those previously published on water chemistry and age
\citep{Chabauxetal2017,Ranchouxetal2021,Ackereretal2021}, allow us
to propose a structural architecture of the deeper zone developed in
the granitic basement of the Strengbach watershed. A consequence would
be to give a first conceptual scheme for the localization and nature of
water circulations at a site representative of HRA in landscapes of
mid-altitude mountains. 

\section{The Strengbach critical zone observatory}

The Strengbach CZO, hereinafter referred to as the Strengbach
watershed, relates to the uppermost part of the Strengbach catchment, a
watershed covering 0.8 km$^2$. Its location is upstream of Aubure village,
${\approx}$15 km from the western border of the Rhine valley on the
western side of the Upper Rhine Graben in the Vosges Mountains
(Figure~\ref{fig1}). 

\begin{figure*}
\includegraphics[scale=0.95]{fig01}
\caption{\label{fig1}Geological setting of Strengbach watershed. (A)
European Cenozoic rift system and location of Vosges Mountains in the
western shoulder of Rhine graben. (B) Simplified terrain map of Vosges
Mountains. (C) Geological map of Strengbach watershed and surroundings
\citep[after][]{Blanaltetal1972,Blanaltetal1970,VonElleretal1975,
Menilletetal1978,Skrzypeketal2008}. SMMZ: Saintes Marie aux Mines
fault zone; BFZ: Bilstein fault zone.}
\vspace*{2pt}
\end{figure*}

The geological context of the Vosges Mountains consists of an
arrangement of elongated major tectonic terranes that collided during
the Hercynian orogeny between ${-}$340 Ma and ${-}$320 Ma 
\citep[e.g.,][and references therein]{Flucketal1991,Lardeauxetal2014, Skrzypeketal2014}. 
These terranes form three NE- to NNE-oriented main
lithotectonic\break domains, known as the Northern, Central, and Southern
Vosges (Figure~\ref{fig1}). From a structural point of view, the Strengbach
watershed belongs to the Central Vosges, which mainly
exposes~migmatites, gneisses, and granites with a complex multiphase
tectonic history from the Hercynian orogeny to the formation of the
current Tertiary--Quaternary reliefs. During the Hercynian collision,
compressive structures induced crustal thickening associated with
strike-slip faulting, followed by gravitational collapse and
extensional tectonics. After a subsidence phase and Mesozoic
sedimentation, the Rhine Graben opening during the Oligocene uplifted
the basement in the rift shoulders. In response to the more recent
Mio-Pliocene phases of alpine convergence, the Hercynian basement has
been further exhumed in the Southern Vosges. As a result of this
geological history, present-day regional bedrock structures trend ENE,
N to NNE, and SW (Figure~\ref{fig1}). During the Quaternary, the Vosges massif
underwent erosion and weathering processes and was affected by
glaciations, which decreased in impact northward together with
topography \citep{Ackereretal2022}. 

The Strengbach watershed is located at altitudes ranging between
880--1150~m asl. The watershed is marked by significant slope
asymmetry, with a gentler north-facing slope (8--9\textdegree) and a
south-facing slope around 15--16\textdegree\ (Figure~\ref{fig2}). The climate is
temperate, oceanic and mountainous with an average rainfall of 1400 mm
yr$^{-1}$. Forest covers 90\% of the area with 80\% spruce
(\textit{Picea abies}~L.) and 20\% beeches (\textit{Fagus sylvatica}). The bedrock is
mainly composed of Hercynian Ca-poor granite, fairly hydrothermally
altered, with small microgranite and gneiss inclusions along the
southern and northern slopes, respectively
\citep[][and references therein]{Pierretetal2018,Chabauxetal2019}.
Compared with granite, gneiss is Mg-rich due to the higher proportion
of biotite and the presence of chlorite 
\citep{ElGhmari1995,Fichteretal1998a,Fichteretal1998b}. The granitic bedrock has undergone late hydrothermal
alterations, which are described as more important on the northern
slope of the watershed than on the southern slope
\citep{Fichteretal1998a,Fichteretal1998b}. The Strengbach watershed was most
probably marked by periglacial events and not fully covered by glaciers
or ice caps \citep{Ackereretal2016,Dehnertetal2010,Heymanetal2013}.
Several meters of stripping at the summit area and colluvium deposits
over the slopes occurred approximately 20 ky ago
\citep{Ackereretal2016,Ackereretal2022}. The bedrock is covered by
${\approx}$80--100~cm-{\ubreak}thick soils ranging from Dystric Cambisol to
Albic Podzol [brown acidic to ochreous brown podzolic, \citealp{WRB2014}]. The
soils usually cover a sandy saprolite, reaching several meters in
thickness in some places \citep{Pierretetal2018,Chabauxetal2019}.
Bedrock outcrops are scarce, except for a few bouldery\break tors. 
\pagebreak

\begin{figure*}
{\vspace*{-3pt}}
\includegraphics{fig02}
{\vspace*{-3pt}}
\caption{\label{fig2}Strengbach watershed and location of the deep
wells (Fx), piezometers (pzx) and pits (Fox). (A) Fracture direction
measured in boreholes (Fx) and pits (Fox) located on simplified
topographic map (contours every 10 m). The background is a slope map
derived from an~aerial 2018 Lidar survey (OHGE data). Knickpoint in
valley bottom is indicated separating the western preserved upper part
of the lower incised part of the watershed. Symmetrical rose diagram of
direction represented with 10\textdegree\ classes, radius corresponds
to 100\% of maximum class, N: number of data; dashed line: direction of
scanlines in the pits. (B) Geological cross-section with location of
boreholes and pits, and fracture distribution along depth for F1a, F1b,
and F5 (cumulative number of fractures vs depth). Bad image quality in
other boreholes prevents relevant fracture distribution analysis.}
\end{figure*}


\section{Sampling and analytical methods}


{The drilling of six deep boreholes (50--120 m depth; F1a, F1b, F5, F6,
F7, F8 in Figure~\ref{fig2}b) and three piezometers (${\approx}$15 m
depth; pz3, pz5, pz7 in Figure~\ref{fig2}b) was performed in
2012--2014. The nine boreholes are distributed along two transects, one
along the south-facing slope and the other along the north-facing slope
(Figure~\ref{fig2}). F1b, F5, and F6 were cored and F6, F7, and F8 were
entirely cased and equipped with multi-level screened sections.
Acoustic and optical borehole wall images identify fracture zones at
depth, leading to the installation of well screens (1.5 mm slots) in
front of these fracture zones, considered as potential water
circulation levels (five screened sections in F6, three sections in F7,
two sections in F8). Sectional drawings of the six boreholes drilled in
the Strengbach watershed and their main construction features are
provided in Supplementary  Section~1. A gravel pack (gravel diameter:
3--5 mm) was set up outside the screened sections to ensure proper
water flow, and clay plugs of bentonite were used to isolate the
screened sections. F5 and F1 remained uncased, except for the first 8
m, where a PVC casing and cement sheath were\break installed.} 

{Physical--chemical loggings were regularly performed in the different
wells with an OTT KL 010 TCM probe over the period 2015--2019 and an
Idronaut Ocean Seven 303 multiparameter CTD probe in 2018. Electrical
conductivity calculated at 25~\textdegree C (}${\upmu}${S/cm) and
temperature (\textdegree C) were always measured, with the latter
probe providing pH, redox potential (expressed in mV in reference to
the potential of the Standard Hydrogen Electrode), and dissolved oxygen
concentration (\% of saturation) profiles in the water column of the
wells. The presentation of the downhole logging approaches used in the
study is given in Supplementary Section~2.1. We also determined the
fracture number and orientation in boreholes F1, F5, F6, F7, and F8
from the analysis of acoustic and optical borehole wall images acquired
in November 2012 and June 2014. Fracture orientations were also
measured along scanlines in the two pits Fo1 and Fo3 (Figure~\ref{fig2}),
previously studied by \citet{Ackereretal2016,Ackerer2017}. The method
is detailed in Supplementary Section~2.2. Rock samples
representative of the principal rock types observed in the cores were
collected along cores F1b, F5, and F6 for porosity and permeability
determination. Methods are detailed in Supplementary Section~2.3.}

\section{Results}

\subsection{Petrological units}

The three cores' macroscopic petrological characteristics are given in
Supplementary Section~3.\break The principal information is summarized
below and in Figure~\ref{fig3} and Supplementary Figure~S1
(Supplementary Section~3).

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}Geological log of the three cored boreholes (F1b,
F5, and F6) with localization of the main fractures and indications of
the main zones of FeOx impregnation.}
\end{figure*}

{The data, associated with those previously published for pits Fo1 and
Fo3 \citep{Ackerer2017,Ackereretal2016}, show that in pits Fo1
and Fo3 and boreholes F1b and F5, the bedrock is porphyry granite. In
F1b, the granite is crosscut by decimetric aplite and pegmatite veins
at 31--35 m and 71--72 m. Core F5 is marked by the presence of a
metamorphic enclave at 30.66 m. F6, located on the northern slope,
first \mbox{crosscut} the gneiss injected by granite down to 42 m depth and
then a porphyry granite similar to that observed on the southern
slope.} 

Based on the petrological description of the three cores given in
Supplementary Section~3, the following main features can be drawn: 

\begin{itemize}
\item The three cores can be divided into a soil horizon with limited
vertical extension (0.8--1.2 m), an unconsolidated saprolite level, and
an in-place fractured bedrock. The latter is subdivided into an upper
level of a few meters with open sub-horizontal fractures that pass with
depth to a more massive bedrock with much less or no visible open
sub-horizontal fracturing. The soil and unconsolidated saprolite
horizons constitute the so-called mobile regolith of the profile. The
upper part of the in-place granite with open sub-horizontal fractures
is usually named the saprock level and is identified as such in this
study. The fractured bedrock term has thus been limited to the bottom
section of the core below the saprock level. 

\item The fractured bedrock is more massive than the saprock with the
presence of macroscopic fracture zones or non-horizontal cataclase
zones of limited vertical extension. The zones of fractures and
cataclases identified in the cores along with their main direction are
located in Figure~\ref{fig3}. These zones evidenced current or recent water
circulation. Outside such zones, the deep bedrock is slightly altered
in appearance. 

\item Saprock and fractured bedrock horizons are characterized by mesoscale
fractures clogged by secondary mineralogical infills. In both levels,
these sealed fractures are intersected by the presently open fractures.

\item The mobile regolith thickness increases along the southern slope of
the basin, from ${\approx}$1--2 m upslope (Fo1) to ${\approx}$9--10 m
downslope, close to the Strengbach stream. The saprock zone is less
developed in core F1b (2--3.3 m) than in core F5 (9--12.5 m). 

\item In borehole F6, located at the top of the northern slope, the
transition between mobile regolith and in-place gneiss of the saprock
horizon is at 2.5 m depth, and the transition between saprock and
fractured gneiss horizons is at 3.3--3.5 m. F7, F8, and pz7, located
along the same hillside, were not cored. However, as detailed in
Supplementary Section~3, some observations suggested a mobile
regolith thickness of ${\approx}$2 m, varying only slightly from top
to bottom along the south-facing slope as opposed to observations from
the north-facing\break slope.

\item Colluvium horizons are observed in pit Fo3
\citep{Ackerer2017,Ackereretal2022} and, as detailed in Supplementary
Section~3, in cores F5 and likely F1b, indicating a relatively large
lateral extension of colluviums on the north-facing slope. The absence
of pits or non-destructive coring along the south-facing slope made it
impossible to assess whether or not the regolith of this hillside is
also dominated by colluvium\break deposits.
\end{itemize}

\subsection{Fracture network: description and orientations}

Borehole wall images are only available below 3--8 m depths, depending
on the wells, essentially within the saprock and fractured bedrock
zone. Borehole imagery combined with core observations shows that the
basement of the Strengbach watershed is affected by a large fracture
network, irrespective of depth (Figure~\ref{fig2}; Supplementary Table S1). Two types of
fractures are distinguished: the meso-fractures, which are thin
fractures with an aperture less than 1 mm, and major fractures, which
present thick or associated thin fractures in fractured zones with a
thickness over 1 cm. 

Most of the meso-fractures have a steep dip (${>}$50\textdegree). They
are present both in saprock and fractured bedrock and are the major
ones in the\break fractured bedrock. Others with a ${<}$20\textdegree\  
dip are considered as sub-horizontal fractures. They are often sealed
by iron hydroxides and occur as sub-parallel ${<}$4--5 cm long and
${<}$1 mm-thick cracks. Open sub-horizontal fractures are mainly
observed in the saprock horizon and only a few in the fractured
bedrock. The largest fractures show mineral infilling, cataclastic
texture, and associated wall-rock alteration (Figure~\ref{fig3}). Orientations of
the largest fractures and of steep-dipped meso-fractures give evidence
of one to two main fracture sets, depending on the considered borehole
(Figure~\ref{fig2}). The main set S1 is oriented N--S to NNE--SSW, and the
secondary set S2 is~oriented NE--SW to E--W. S1 has an orientation
similar to the direction of the Upper Rhine Graben (Rhenan direction),
whereas S2 has an orientation similar to the Hercynian terrane
directions (Figure~\ref{fig1}c). These two main fracture directions are thus those
of the regional tectonic orientations inherited from the geological
history of the massif.

\subsection{Porosity and permeability}

Porosity and permeability were measured in core samples from the
boreholes F1b, F5, and F6 (Supplementary Table S2; Figure~\ref{fig4}). The data pointed out a
general decrease in porosity with depth. Porosity decreases from 20\%
to 0.6\% in F5 when depth increases from 6 to 47 m. Even if the feature
is less pronounced and assigned larger data dispersions, porosity
decreases from 19\% to 1.6\% in F6 when the depth increases from 14~m
to 118 m. The range of porosity variation was smaller in F1b, dropping
from 5\% to 1\% as depth increased from 2~m to 86~m, except at 30 m
depth, where a singular high porosity value was measured. The
relationship between permeability and depth is similar to that of
porosity (Figure~\ref{fig4}). Permeability decreases by three orders of magnitude
as depth increases from 6~m to 47~m in F5. There is a similar decrease
for F6 but over the whole depth range (ca.~100 m) and if the data
corresponding to the fracture zones (${\approx}$60~m) are not
considered. Finally, there is almost uniform permeability over the
investigated depth range for F1b, with two exceptions at 20 m and 86 m.
The fracture measurements on the pit outcrops Fo1 and Fo3 indicate that
the dominant fracture sets are roughly NE--SW (N20\textdegree\ 
E--N60\textdegree\ E) and E--W (N80\textdegree E--N90\textdegree\ E)
oriented in Fo1 and NNE--SSW (N0\textdegree\ E--N30\textdegree\ E) in
Fo3 (Figure~\ref{fig2}b).

\begin{figure*}
\includegraphics{fig04}
\vspace*{-1pt}
\caption{\label{fig4}Depth variation of porosity and permeability. 
For F5, the porosities decrease progressively down to 14 m depth
(${\approx}$ bottom of the saprock horizon) to reach low values at 20 m
depth and beyond, except in the vicinity of macroscopic fracture or
cataclastic fractured zones. In F1, the low saprock extension logically
results in low porosity and permeability from 3--4 m depth. In F6, high
porosity values are measured well below the saprock level, i.e., up to
31 m depth, which is in line with the high degree of fracturation of
the gneiss bedrock horizon.}
\vspace*{-1pt}
\end{figure*}

\subsection{Physical--chemical borehole fluid logging}

Several physical and chemical waterlogging campaigns have been
performed since 2016, following the protocols given in 
Supplementary Section~2.2. The
majority of the loggings were carried out under natural flow conditions
and only a few after water extraction by pumping. The results of these
logs are described below for borehole fluid temperature and electrical
conductivity under natural flow conditions and after pumping.

\subsubsection{Temperature}

Temperature profiles measured under natural flow conditions in
boreholes and piezometers (Figure~\ref{fig5}) show seasonal variations down to
20--25~m depth, whether the well is cased or not. Borehole F6, with its
deeper water table level, is an exception as it was only slightly
influenced by seasonal changes. Below 20--25 m depth (40 m depth for
F6), temperatures were much more stable over time and increased with
depth according to the regional geothermal gradient. These profiles
reflect water columns in thermal equilibrium with their local
environment and therefore indicate that water inflows are not
sufficiently large to disrupt these temperature profiles at depth.


\begin{figure*}
{\vspace*{-3pt}}
\includegraphics{fig05}
{\vspace*{-3pt}}
\caption{\label{fig5}Borehole fluid temperature loggings using the OTT
KL 010 TCM probe.}
\end{figure*}

\subsubsection{Fluid electrical conductivity}

{The borehole fluid electrical conductivity profiles are given in 
Figure~\ref{fig6}. They are more contrasted than temperature profiles. A first group of
boreholes includes F6, pz5 and pz7, where the electrical conductivity
does not vary with depth, or slightly at the well as a result of the
dissolution of accumulated material \citep{Ranchoux2020}. Changes over time
in electrical conductivity are limited everywhere. A second group
includes all the other boreholes and pz3. Conductivity profiles show
one or several increases with depth, e.g., from values near 30}
${\upmu}${S/cm (at 25~\textdegree C) from 2 to 12~m depth up to
${>}$200} ${\upmu}${S/cm from 12~m to 15 m in pz3. Changes of similar
amplitude were observed in F5, with sudden increases varying in depth
as a function of time, shallower when the static level was shallow (6.3
m and 12 m respectively),
and deeper when the static level was deeper
(11.6 m and 23 m depth, respectively). However, the interface depth is
not a direct function of the water table level evolution. As F5 is
largely uncased, such conductivity profiles suggest the existence of at
least two contrasted water inflows along the borehole, with
low-conductivity water similar to that measured in spring water at the
surface and higher-conductivity water in the deeper part of the
borehole.} 

\begin{figure*}
{\vspace*{-3pt}}
\includegraphics{fig06.jpg}
{\vspace*{-3pt}}
\caption{\label{fig6}Borehole fluid conductivity loggings using the OTT
KL 010 TCM probe.}
\end{figure*}


For F1b, when the static water level is high, an interval of higher
conductivity often appears at the top of the water column. It is
attributed to concrete leachate (see Supplementary Section~4). The
conductivity profiles in F1b also suggested that this higher level
progressively migrated downhole over time and was therefore not diluted
by lateral flow. This implies a low rate of water production in this
borehole. This is confirmed by the weak local hydrological connection
between F1b and F1a, 6 m away from each other, indicated by the absence
of any significant variation in the piezometric level in F1b when F1a
was purged. The conductivity in F1b is low compared to that measured in
the deep F5 levels, but systematically higher than that of spring water
or piezometer water.\break Moreover, two to three different conductivity
zones were observed during some investigations, with conductivity
increasing with depth (Supplementary Figure S2 in Supplementary Section~4),
suggesting the occurrence of different deep-water inflows in F1b. 


Contrary to F1b and F5, F6, F7, and F8 were equipped with a succession
of cased and multi-level screened sections (Supplementary 
Section~1). Their water conductivity was high, e.g. ${>}$120 ${\upmu}$S/cm
for the lowest values of F7 to approximately 260--270~${\upmu}$S/cm
in F6 or the highest water conductivities in F7 (Figure~\ref{fig6}). There are no
low conductivity horizons and thus no surface waters infiltration in
these boreholes due to the casing of the upper part. Only F6, equipped
with four screened sections, has relatively uniform conductivity
profiles throughout the water column. Such an observation can indicate
that this borehole is mainly supplied by the water flow \mbox{identified} at
63 m depth during drilling and with no other significant contribution
from other horizons. Borehole F7, equipped with three screened
sections, has a conductivity profile marked by several steps,
suggesting the existence of several water-productive horizons along the
borehole, with at least two different production zones between 40~m and
47 m. Borehole F8, with two screened sections, shows small
conductivity changes with depth that occurred at a depth between 25~m
and 30 m (Figure~\ref{fig6}), suggesting the occurrence of at least one specific
water supply in each screened section. 

\subsubsection{Influence of pumping on physical--chemical logs}

The influence of pumping on the temperature and chemical profiles of
the water column was evaluated by performing physical--chemical logs
just after a pumping session. The results are compared to the logging
results before pumping (Figure~\ref{fig7}), the latter being considered
representative of equilibrium conditions under natural flow. In
addition to a decrease in the water table level, indicative of
relatively low-productive water horizons (pumping rate 
${<}$1.5~m$^{3}$/h), the temperature and chemical profiles
change in all boreholes and depart from equilibrium conditions, with
the occurrence of some anomalies. When the temperature peak is at the
pump level, an artifact induced by the local pumping stress cannot be
avoided (e.g., heating induced by the pump rotor). However, when
located at different depths or associated with other chemical changes,
the temperature anomalies are better related to water inflow from
productive zones. In our study, temperature anomalies were often
associated with conductivity anomalies, as shown in Figure~\ref{fig7}. For
example, this is the case for F6 at 62--63 m depth, where an increase
of more than 0.5~\textdegree C is measured. Second-order changes are
observed from pH or dissolved oxygen concentrations. Based on such
anomalies, a limited amount of water circulation zones are identified.
These zones are systematically associated with fracture zones
identified on the cores and borehole wall images. These zones are
listed in Table~\ref{tab1}.

\begin{figure*}
\includegraphics{fig07}
\caption{\label{fig7}Comparison of borehole fluid physico-chemical logs
(temperature, electrical conductivity, pH, and dissolved oxygen using
the Idronaut probe) prior to and after pumping. The geometry of the
boreholes is indicated by the technical sections. The main productive
water horizons are indicated by arrows.}
\end{figure*}



\begin{table*}
\caption{\label{tab1}Water flow levels determined from the
physico-chemical loggings in the Strengbach watershed boreholes and
relation with geological features deduced from borehole imagery and
core observations\vspace*{2pt}}
\fontsize{9.5}{11.5}\selectfont
\tabcolsep=3pt
\begin{tabular}{ccll}  
\thead
Borehole & \parbox[t]{3cm}{\centering Water flow
depth interval (m)*}\vspace*{2pt}  & Used
physico-chemical criteria  & Geological
observation \\ 
\endthead
Pz3 & 11--12 & Temperature and redox & No available observation \\ 
F5  & 15--16 & Electrical conductivity & NE--SW and E--W fractures \\ 
F5  & 20--24 & Dissolved oxygen  & E--W fracture zone \\ 
F5  & 30--33 & Temperature & Lithological discontinuity at 33 m \\ 
F5  & 43--44 & Electrical conductivity & E--W fracture zone at 45 m \\ 
F6  & 62--63 & \parbox[t]{5.4cm}{Temperature, electrical conductivity\newline and dissolved oxygen} & \parbox[t]{5cm}{Major fracture zone at 63.5--64.3 m\newline (N72\textdegree\ E direction)}\vspace*{2pt} \\ 
F6  & 77--80 & Dissolved oxygen & Large N--S fracture at 79 m  \\ 
F6  & 110    & Temperature & \parbox[t]{5cm}{NW--SE to E--W fracture zone\newline between 113.3--114.4 m}\vspace*{4pt} \\ 
F7  & 41--42 & Electrical conductivity & NE--SW single fractures \\ 
F7  & 46     & \parbox[t]{5cm}{Temperature, pH and electrical\newline conductivity}\vspace*{2pt} & NE--SW single fractures \\ 
F8  & 21     & \parbox[t]{5.3cm}{Dissolved oxygen and electrical\newline conductivity}\vspace*{2pt} & E--W (N72\textdegree\ E) fracture at 20.7 m depth \\ 
F8  & 27--28 & \parbox[t]{5.3cm}{Electrical conductivity and\newline temperature}\vspace*{2pt} & NE--SW fracture zone between 25--29 m \\ 
F8  & 43--47 & Dissolved oxygen & N55\textdegree\ E single fracture 
\botline
\end{tabular}
\tabnote{* Depth interval as suggested by changes noticed on
physico-chemical logs when comparing log under natural flow conditions
and log shortly after pumping.}
\vspace*{2pt}
\end{table*}

Physico-chemical logging, performed sequentially after pumping,
provides data on the return time to equilibrium. Figure~\ref{fig8} shows such
acquisitions, performed only in F5, which is open below 9~m (i.e.,
above the static water level). All the water column data correspond to
flow conditions in open holes, with lateral flows coming from
productive levels and eventually from vertical flows induced by
temperature and/or salinity gradients. Three reference logs acquired
under undisturbed conditions (natural flow conditions before pumping)
are presented for April 2018 and June/July 2019 (Figure~\ref{fig8}). 


\begin{figure*}
\includegraphics{fig08}
\vspace*{-1pt}
\caption{\label{fig8}Borehole fluid physico-chemical logs from the
Idronaut probe in F5, presenting temperature, electrical conductivity,
pH, dissolved oxygen, and redox changes prior to and after pumping.}
\vspace*{-1pt}
\end{figure*}


Pumping actions performed without packers severely disturbed the
natural flow conditions, as shown by the conductivity log performed
shortly after pumping on April 24, 2018. This perturbation was not
rapidly resorbed, as highlighted by the conductivity log recorded two
days later (April 26). Thus, several days were needed to recover
steady-state conditions, as also evidenced by \citet{Ranchoux2020} for
similar monitoring made in 2015. A similar statement can be proposed
regarding pH and even dissolved oxygen. The situation is different with
temperature. The effect of pumping is shortly transient, probably
because the fluid temperature is quickly buffered by the rock
temperature. Perturbations lasting several days also suggest quite a
reduced water production from the different production levels.

\vspace*{-3pt}
\section{Discussion}

The results from the Strengbach CZO lead to a comparison of the
architecture of the watershed bedrock with the nature of the water
flowing through it. Such a combined dataset, still scarce at this
scale, is relevant to discuss the aquifer and water circulation level
pattern in this watershed, representative of mid-mountain granitic
basins.

\vspace*{-3pt}
\subsection{Nature and vertical extension of the surface aquifer}

Structural and petrological observations of the cores and boreholes
combined with observations of the pits give a first view of the
underground vertical structure in the Strengbach watershed. It consists
of a loose or mobile saprolite, on top of which the soil develops and
under which an in-place fractured bedrock zone extends over depth. The
in-place bedrock is subdivided into a few meters of extended saprock
level characterized by open sub-horizontal fractures that pass with
depth to a more massive bedrock (the fractured bedrock level) with few
or no visible open sub-horizontal fracturing. Nowhere is a level of
fresh bedrock reached without evidence of water circulation.

Water conductivity data evidence that the piezometer and surface waters
from F5 have much lower conductivities than the F5 deep waters.
Moreover, their conductivities are close, not to say identical, to
those of spring waters. Such an observation indicates that both water
types circulate in the same aquifer system. The very similar
geochemical and isotopic data of these waters
\citep{Chabauxetal2017,Ackereretal2018,Ackereretal2020,
Ranchouxetal2021},
as well as the results of the modeling of
the geochemical composition of the spring waters and the piezometers of
the south-facing slope \citep{Ackereretal2020}, lead to the same
conclusions. All these waters circulate within the same hydrological
compartment corresponding to the surface aquifer controlling the spring
waters' chemical composition. Comparison of hydrological data with core
and well data helps clarify the nature and vertical extent of this
aquifer.

Borehole F5 was drilled in a 9 m-thick mobile regolith zone and a 3--4
m-thick saprock zone. It was sealed with PVC and cement pipe to a depth
of 8--9 m. Borehole F5 can, therefore, only be fed by\break water circulating
in the lower horizons of the mobile regolith or deeper. Observation of
water of low conductivity in the upper part of the water column to
depths up to 13 m depth or more, depending on the season and in the
absence of wellbore flow, suggests that the water in the so-called
shallow subsurface zone may still circulate below a depth of 10 m, that
is, in the saprock. The measurement of very high porosity and
permeability at 10 m depth and still high at 12--15 m depth in core F5
is consistent with this conclusion. The conductivity profiles measured
in pz5, always with low values of the spring water type, are also
consistent with this scheme. The pz5 was drilled close to F5 down to a
depth of 15 m, with a sealed casing over the first two meters, thus a
priori only intersecting the mobile regolith--saprock level. In the core
from F5 below these depths and particularly below 20 m, the bedrock is
characterized by low porosity and permeability measurements, apart in
the close vicinity of fracture zones, suggesting a different water
circulation scheme in the fractured bedrock zone, as detailed in
Section~\ref{sec5.3}. Therefore, based on these\break different observations, the entire
saprolite--saprock system can be interpreted as representing the
watershed's shallow aquifer in the Strengbach critical zone\break
observatory.

{Field data show a thickening of the saprolite-mobile saprock system
along the north-facing slope of the watershed and an a priori thinner
thickness on the south-facing slope, at least in the vicinity of F7 and
F8. Assimilation of this zone to the subsurface aquifer would,
therefore, imply similar characteristics for the aquifer, which
hydrogeochemical data confirmed. Pz3, set up at the mid-slope of the
\mbox{north-facing} slope, is located in a thinner saprolite--saprock zone than
F5. Sealed over its first two meters, pz3 collects water flowing
between 2 and 14 m depths. The conductivity profiles show low values
(${\approx}$30 $\upmu$S/cm) of spring water-type up to
${\approx}$10 m depth and more conductive water deeper down. This
suggests that this piezometer might intercept the deep circulation
system in its lower part. The electrical conductivity profiles carried
out after pumping in the piezometer pz3 confirm this characteristic as
they show a fracture-type water inlet at 11--12 m depth (Figure~\ref{fig6}).
Borehole F1b is drilled further up the slope where the mobile regolith
is thinner than in pz3 and is watertight over its first 8~m depth,
i.e., over the entire zone of mobile regolith and saprock section of
the core (Figure~\ref{fig3}). Therefore, this well would only drain water
circulating in the fractured bedrock. The higher water conductivities
in F1b (${\approx}$50} ${\upmu}${S/cm or above) than in spring water
and piezometers (20--30} ${\upmu}${S/cm) certainly validate that the
waters in F1b are not part of the shallow circulation system but of one
with a deeper circulation. The observation of low porosity and
permeability in samples from F1b (Figure~\ref{fig4}) is consistent with this
pattern since the samples used for these analyses were taken at depths
of 2 m and more, thus essentially outside the shallow aquifer,
explaining their low porosity and permeability values.}

On the south-facing hillside, the casing equipment of F6 and F8 (see
Supplementary Section~1) only allows for sampling waters below the
saprock. Only pz7, sealed over the first 2 m depth, allows for surface
water sampling. Low water conductivity measurements in pz7, similar to
the conductivity of water from other piezometers and spring water,
indicate that this well is mainly supplied by shallow subsurface water.
The flow rate through this well seems much lower than in pz3 and pz5,
based on the very rapid emptying of the well compared to the other two
during pumping operations. This would imply a relatively weak
circulation for the saprolite not occluded by the casing, consistent
with the suggestion that on this hillside, at least at the location of
F7 and F8, the saprock's vertical extension is lower than on the
opposite slope. Such a configuration would also imply that F7, uncased
between 8~m and 18~m depth, is not supplied by surface water, as
confirmed by the absence of any low conductivity water level
measurement in this well, unlike F5.


All these observations confirm a pattern of meteoric water flow in a
shallow aquifer, formed by\break mobile saprolite and saprock, with
reasonably strong lateral variability in aquifer thickness at the
watershed level. A more accurate geometry of the subsurface aquifer at
the watershed scale will certainly be necessary to model hydrological
flows at this scale. This will undoubtedly be achieved through
geophysical investigation methods. Our results are interesting in
providing a relatively simple and sound scheme for the vertical
structure of the subsurface aquifer for the Strengbach watershed. This
scheme implicitly assumes or confirms that the saprock level marked by
subhorizontal open fractures certainly represents the propagation zone
of the surface weathering front in the bedrock and thus marks the lower
limit of the shallow aquifer of the Strengbach watershed. 

\subsection{Spatial variation of the surface aquifer thickness}

Our data suggest a difference in the regolith thickness depending on
slope orientation, as well as along the north-facing slope. Watersheds
under geomorphological equilibrium or steady-state forcing conditions
usually have greater mobile regolith thicknesses near the summit than
in the talweg \citep{RempeDietrich2014}. This was not observed on the
north-facing slope and may suggest that the watershed is far from
equilibrium. The Lidar Digital Terrain Model (DTM) of the Strengbach
watershed (Figure~\ref{fig2}) and, especially, the derived slope map highlight the
recent V-shaped stream carving of the valley bottom, incising the
gentler slopes of the upper parts of the catchment. The well-defined
slope breaks surrounding the deepest part of the valley suggest a
regressive erosion of the Strengbach stream and that the watershed is
not in a geomorphological steady state but on the way to
re-equilibration. $^{10}$Be data validate such an
interpretation \citep{Ackereretal2022}. This interpretation implies
that the watershed can be subdivided into two sub-zones. First, a zone
above the knickpoint, preserving the topography inherited from erosion
processes that occurred during the last glacial maximum and stabilized
afterward. Second, a zone of active erosion below the slope break where
the V-valley morphology is observed. Upstream and west of the system,
where pits and boreholes F1--F5 are located, the watershed is a large
valley with gentle slopes (8--9\textdegree) with little headward
erosion. Downstream and further north, primarily where F7 and F8 are
located, the valley is narrower with steeper slopes
(15--16\textdegree) in response to stream carving. Based on these
different observations, we propose to explain the differences in slope
geometry and structure between the two hillsides by different erosion
rates or processes between the two watershed slopes based on the fact
that the Strengbach watershed area has not yet reached a
geomorphological steady state.

\subsection{Deep water circulations~and spatial variations of water conductivity}\label{sec5.3}

The loggings in the fractured bedrock below the saprock horizon,
especially those after pumping stimulations, identify specific and
well-localized water inflows along the cores. When comparing the
location of the water inflows to the description of the available cores
(F1--F5--F6) or wall images, they are found to be systematically
associated with or close to well-identified fracture zones or
discontinuities (Table~\ref{tab1}). These data, therefore, identify in the deep
part of the bedrock a specific water circulation in the macroscopic
fracture zones riddling the bedrock. Nevertheless, at this stage, it is
not possible to eliminate a matrix circulation in the deep zone outside
the macroscopic fracture zones. If such a matrix circulation exists,
our data point out, however, that it would be much weaker than the
circulations in the fracture network and undetectable based on the
measurements presented in this work. 

{Our data also suggest a systematic variation of deep-water
conductivity with water circulation depth in granitic bedrock. In F7
and F8, cored on the south-facing slope and only collecting water
circulating in the fractured granitic bedrock, an increase with depth
of water conductivity (150--205} ${\upmu}${S/cm in F7 and 170--200}
${\upmu}${S/cm in F8) was observed. Such a systematic variation can
suggest that flow paths or water travel times are longer when water
circulates deeper in the bedrock. On the north-facing slope, F1b and
F5, also drilled in the granitic bedrock, are open along their whole
length except the upper 8~m. The equilibrium conductivity profiles of
F5 imply feeding by a highly conductive deep water (Figure~\ref{fig6}). Pumping
experiments in F5 suggest circulation levels at ${\approx}$23 and
${\approx}$32 m depth (Figure~\ref{fig7}), with intermediate conductivities
between those of deep water and surface water, even if it is difficult
to precisely characterize them. In F1b, for periods assumed to be free
of pollution problems (details in \mbox{Supplementary} Section~4), two or
three circulation levels with different and increasing conductivities
from surface to depth can also be considered. Thus, these data follow
the logic observed on the opposite slope, indicating that for the
entire granitic basin, the deeper the fractures, the longer the path
and/or the longer the water transit time in the fractures. With that
kind of scheme, the lower water conductivities in F1b compared to the
deep water in F5 would indicate that borehole F1b at the top of the
slope is supplied with water of relatively short transit times compared
to the F5 deep-water borehole. This would be logical in a context fed
with local water, i.e., waters simply percolating from the catchment
area and not imported by subsurface flow from farther areas. Only the
water from borehole F6, cored at the top of the south-facing slope and
mainly fed by a fracture at 63 m depth, does not follow the above
pattern. Deep water in F6 is more conductive than that in F7 and F8,
located further down the slope. It is tempting to link this difference
to the lithological variability of the watershed, particularly as F6
intersects a significant thickness of gneiss, unlike the other wells in
the watershed. Nevertheless, it cannot be excluded that this well,
marked by a vast cataclase zone, could drain water from outside the
watershed, unlike the other wells, even if the hydrogeochemical
modeling of the deep water does not support such a scenario
\citep{Ackereretal2021}.}

Global water fluxes transiting through deep circulations cannot be
quantified on the basis of our data. However, qualitative information
can be derived from the partial emptying experiments of F5 (Figure~\ref{fig8}). An
extension of the low conductivity level toward the lower part of the
water column during the borehole emptying, whatever the pumping depth,
indicates that systematically, the water produced by the deep horizon
was preferentially abstracted during pumping and was not renewed at a
rate corresponding to that of the pumping. This suggests that the
production of more mineralized water from the deeper part of the well
is low and that pumping induces the invasion of bottom-holes by less
mineralized waters from shallower horizons. Such an observation is
easily explained in the context of surface circulation marked by more
intense water fluxes than in deep-water circulation systems. Similarly,
the fact that it took a few days to bring the disturbed chemical
profiles to an equilibrium profile, marked by a high electrical
conductivity level rising to its pre-pumping level, indicates a lower
intensity of the deeper water circulation than the subsurface one. A
low circulation intensity in the deep fracture zones compared with the
surface circulation is also consistent with the difficulty of detecting
these flows by flowmetry for all wells. These methods could only
identify a flow in the fractured zone associated with cataclastic
textures in F6 at a depth of 63 m by identifying a low and not
quantifiable intensity flow just slightly above the background noise of
the flowmeter. Based on these data, we propose a circulation in the
deep bedrock mainly through the fracture network that structures it,
with lower intensities (this study) and much slower circulation times
\citep{Ranchouxetal2021,Ackereretal2021} than those estimated for
waters in subsurface aquifers. Our data show that the fracture network
is developed along regional tectonic orientations inherited from the
region's geologic history. Thus, it corresponds to a pre-existing
fracture network, partially reopened. These results highlight the need
to consider the inherited geological structures to understand the water
circulation pathways in the deep CZ and, more generally, the structure
and functioning of the CZ. The data also question the cause of the
fracture opening. Is it an opening of clogged fractures that
progressively propagate via the action of water infiltrating them? Or an
opening prior to the circulations that secondarily offer a path to the
shallower waters infiltrating them? Our data are inconclusive, but the
hypothesis of a reopening of the fractures in response to rock
decompression during the development of the Tertiary and Quaternary
Vosges reliefs, or simply in connection with erosion processes, is
entirely conceivable. In such a scenario, the vertical extension of
these open fractures and thus of the deep circulations they control in
the bedrock could be much larger than the few tens of meters
investigated by the boreholes. It may imply a water outlet much further
downstream in the \mbox{valley} than the Strengbach CZO outlet. Independent of
this point, our data also question the significance of this deep water
with respect to the critical zone: is the deep water still influenced
by surface circulations and therefore does it still belong to the
critical zone? Or is ancient water that is decoupled from current
surface processes and/or has another origin (marine, glacial\,\ldots)
as seen in deep hard~rocks  \citep{Sanjuanetal2016, Negreletal2001,
Negreletal2018} or in more shallow waters as observed in the Armorican
massif for instance \citep{Aquilinaetal2015}. In the case of the
Strengbach deep waters, our results and interpretation suggest that
deep waters are fed by waters simply percolating from the catchment
area. Water dating using CFC methods \citep{Ranchouxetal2021} shows
ancient ages for the Strengbach borehole deep waters. However, these
initial results do not allow us to choose between a pattern of ancient
waters disconnected from surface waters and a pattern of mixing between
ancient waters and present-day surface waters. Further investigations
are needed to constrain the precise degree of connectivity of the deep
waters with the surface waters, and therefore whether they should be
considered as belonging to the critical zone or not.

\section{Conclusion}

This study integrates geological, petrophysical, and hydrogeological
information from six 50--120 m-depth boreholes, complemented by three
15 m-depth piezometers drilled at the Strengbach Observatory site. The
observation of the boreholes, in addition to the analysis of cores
recovered for three of the six deep boreholes, renders the first
detailed picture of the structure/architecture of the first ${\approx}$100~m 
of the CZ at the scale of the Strengbach CZO. The main
features of hydrogeological functioning are characterized, and a simple
conceptual model of water circulation can be proposed. It includes
shallow water circulation within an aquifer corresponding to the
geomorphological mobile saprolite horizon and the upper part of the
saprock. This shallow circulation superimposes a deeper circulation in
the fractured bedrock via the fracture networks riddling this horizon.
The data show that the vertical extension of the saprolite and saprock
horizons, and thus of the shallow aquifer, differs for the two
hillsides. We propose to relate these differences to an evolving
erosion regime between the upstream and \mbox{downstream} portions on each
side of a knickpoint in the main stream bed. The data also show that a
deep bedrock level without evidence of water circulation is never
reached even below 100 m depth. They also provide~evidence that deep
water circulations in the fractured bedrock horizon are controlled by
fractures of regional orientation inherited from the geological history
of the Vosges massif, integrating the Hercynian orogeny and the
Tertiary and Quaternary tectonics. All these results show the
importance of accounting for the geological heritage of the rocks.
Correctly modeling the hydrogeochemical functioning of the CZ at the
scale of a watershed requires an understanding of the bedrock
emplacement's long-term geological history. It is also necessary to
consider in detail the more recent history of the Quaternary erosion
and alteration, incorporating it into the overall view of the system.

\section*{Declaration of interests}
The authors do not work for, advise, own shares in, or receive funds
from any organization that could {benefit} from this article, and have
declared no affiliations other than their research organizations.

\section*{Acknowledgments}

The authors warmly thank the Guest Editors, especially Val\'{e}rie
Plagnes, for their advices and suggestions, which greatly helped to
improve the article. They also thank the three anonymous reviewers for
their constructive comments and suggestions. This study was financially
supported by the French ANR program under grant agreement
ANR-15-CE06-0014 (Project CANTARE-Alsace). Core drillings were funded
by the REALISE program (CPER Etat Alsace 2003--2014) along with a
funding contribution from INSU-CNRS and another one from the French
Equipex program CRITEX. The contributions of the two OHGE
responsables (M.~C.~Pierret and S. Cotel) are acknowledged.

\CDRGrant[ANR]{ANR-15-CE06-0014}

\back{}

\section*{Supplementary data}

Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.  

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crgeos-239-suppl.pdf}}
\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{Table-S1.xlsx}}
\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{Table-S2.xlsx}}

\bibliographystyle{crgeos}
\bibliography{crgeos20220873}
\refinput{crgeos20220873-reference.tex}

\end{document}
