Granitic Watersheds Research Articles

Influence of Critical Zone Architecture and Snowpack on Streamflow Generation Processes: A Mountain‐Meadow Headwater System in a Mediterranean Climate

AbstractObservations from a granitic watershed within a Mediterranean climate reveal the hydrologic and critical zone functioning of a perennial stream headwater and its upslope contributing area within a meadow system in the Sierra Nevada, California. Chemical analysis (diagnostic tools of mixing models, end member mixing analysis, tritium, etc.) and physical data (stream stage, piezometers, soil water, snowpack, etc.) indicate there are two primary pathways of water input into a headwater stream sourced from a mountain meadow. One input is a shallower and younger subsurface pathway with water that resembles snowpack chemistry, and the other a deeper and older subsurface pathway with water that reflects the chemistry of the groundwater derived from the contributing hillslopes. Multi‐year observations reveal that regardless of snowpack amount, during the period of peak hillslope infiltration, shallow and deep pathways in the hillslope behave similarly to initiate headwater streams. However, during summer dry periods, similarities in active pathways within the meadow center are not maintained between high and low snowpack years. With less snow, perennial groundwater discharge within the meadow center is eliminated, becoming only a seasonal source at the meadow's outlet. At the meadow's edge, geophysically observed downslope thinning in saprolite thickness creates reduced lateral transmissivity and initiation points for headwater streams via enhanced groundwater discharge of upslope water. Combined, these findings suggest how loss of snowpack and critical zone structure can together mediate hydrologic function in a wet meadow system in a Mediterranean climate. Creating new understanding about the stability of hydraulic functioning in headwater wet‐meadow systems under a changing climate.

Improving our ability to model crystalline aquifers using field data combined with a regionalized approach for estimating the hydraulic conductivity field

Modelling of heterogeneous aquifers, such as crystalline aquifers, is often difficult and, flow and transport predictions are always uncertain, suffering of our imperfect knowledge of the spatial distribution of aquifer parameters. This paper aims to test the robustness of a first-order hydraulic conductivity map estimated from both a detailed water-table map and hydraulic-conductivity statistics on a granitic watershed in Brittany (57 km2), compare it to local estimates and assess it through various numerical models. Map values range on four orders of magnitude (10-7 to 10-4 m/s, mean: 3.9x10-6 m/s) and their comparison to local estimates from various sources (pumping tests, MRS measurements, streamflow) gives satisfactory results.Four hydraulic conductivity fields were assessed through numerical modelling under steady-state condition. Model 1 used the regionalized hydraulic conductivity field directly, Model 2 used a uniform value (average of Model 1), Model 3 used a hydraulic conductivity field obtained by inverse modelling of the water table and Model 4 used two zones of uniform values based on the analysis of Model 1 and Model 3 fields. Model results were analysed based on their ability to reproduce the observed water-table levels and the groundwater flow directions. Modelled groundwater discharges to streams at sub-catchment scale were compared to spatial streamflow measurements performed during low water condition, which help validating models. The comparison between the fields obtained from Model 3 and that from the regionalized method (Model 1) shows that they are close in terms of mean values and spatial distribution. Model 1 reproduces rather well the water-table map and the groundwater flow directions. Model 2 shows the less good results. Model 4 has led to satisfactory results and shows that the hydraulic conductivity is higher (2.1x10-6 m/s) where the water table is located in the fractured zone, and lower (3.3x10-7 m/s) where it is located in the saprolites (highly weathered rock), which is expected for such aquifer system. Modelled groundwater discharges to streams are comparable in all models to streamflow measurements in most sub-catchments, but the models overestimate them in certain places, mainly because of sub-surface drains in a forest capturing part of the groundwater that can no longer return to streams (drains were not considered in the models).In addition, experiment on a second watershed (40 km2) shows how with much less field data the methodology can already provide interesting information on the hydraulic conductivity field (values and spatial distribution).Results are very encouraging and open up prospects for using quantitative and qualitative information from the mapping of hydraulic conductivity to constrain the spatialization of hydrodynamic parameters on models and thus our ability to model such complex aquifers.

Chemical weathering rates of clastic sedimentary rocks from the Paraná Basin in the Paulista Peripheral Depression, Brazil

The chemical weathering plays an important role in the processes of surface geochemistry, which is fundamental to understand the landscape evolution. The Paraná Basin is an intracratonic sedimentary basin developed on the South American platform from Paleozoic to Cenozoic, with area of 1.5 × 106 km2 and 5000 km3 of clastic sediments and igneous rocks. The main clastic sedimentary rocks in the Paraná Basin are sandstones and mudstones. Surprisingly, there are no studies assessing the chemical weathering rates of clastic sedimentary rocks from the Paraná Basin under tropical climate conditions in Brazil or in South America. Thus, two small watersheds were chosen to assess the chemical weathering rates of clastic sedimentary rocks from the Paraná Basin in the Paulista Peripheral Depression (São Paulo State, Brazil), i.e. the Monjolo Grande and Jacutinga stream basins. The chemical composition of rain and surface waters, as well as the chemical and mineralogical composition of the fresh rocks (sandstones and mudstones) and soil (horizon C) were characterized. The instantaneous daily flux indicates that ca. 60% of the dissolved solids loads are carried from October to March in both watersheds. The natural fluxes due to chemical weathering (Fw) were ca. 4.5 t km−2 yr−1, with a mixed of local rainwater and chemical weathering of sandstones and mudstones controlling the surface waters composition. The possible chemical weathering processes of sandstones and mudstones occur by quartz dissolution and partial hydrolysis (monossialitization) of albite, microcline and illite, respectively. The chemical weathering rates were of 0.3 and 0.8 m Myr−1 for the Monjolo Grande and Jacutinga Streams basin, respectively, confirming the lower chemical weathering rates for clastic sedimentary rocks in relation to basalts (Paraná CFB) of the Paraná Basin and for the igneous and metamorphic rocks of the Ribeira Belt, other important geological units in the São Paulo State. In addition, these chemical weathering rates are also lower than the suggested for other rock types elsewhere, i.e., carbonates, basalts, granites, schist, gneisses or alkaline rocks. Finally, the effect of climate on [SiO2] was demonstrated, also described in basaltic and granitic watersheds, suggesting that the chemical weathering of sandstones and mudstones are more effective in the wet season.

Chemical weathering of a granitic watershed: coupling Lithium isotopes and reactive transport modeling, preliminary results

In the present study, we report lithium concentrations and Li isotopic compositions for different samples within a granitic watershed (Margeride, France). We investigate unweathered bedrock and samples displaying different stages of weathering in order to characterize Li isotopic fractionation. This was achieved by coupling lithium isotope geochemistry and reactive transport modeling during granite weathering at the scale of this watershed. The following manuscript reports methodology and the preliminary data.

Silicate weathering rate and its controlling factors: A study from small granitic watersheds in the Jiuhua Mountains

Silicate weathering is intimately linked to global climate. To investigate the controlling factors of silicate weathering rate, we collected monthly or half-monthly river water samples from ten small granitic watersheds in the Jiuhua Mountains. Mass balance calculation shows that agricultural activity, atmospheric input, silicate weathering, carbonate weathering and evaporite dissolution contribute 2.0%, 16.9%, 58.7%, 13.5% and 8.9% cations to the river water, respectively. The disproportionate contribution of carbonate rocks with an exposed area of <5% demonstrates the importance of trace carbonate minerals in these small granitic watersheds. The average silicate weathering and CO2 consumption rates at our sites are 7.0 t km−2 y−1 and 3.4 × 105 mol km−2 y−1, respectively.There is no correlation between silicate weathering rates (SWR) and the suspended solid concentrations (an index of physical erosion rate), but there is a strong positive correlation between SWR and temperature, consistent with the kinetic-limited chemical weathering regime in alpine areas. A compilation of literature data from global small granitic watersheds in different climatic zones shows positive correlations between temperature, runoff and SWR. A multiple linear regression between SWR, temperature and runoff yielded an adjusted R-squared of 0.61, showing that climatic factors can account for 61% of the variation in SWR. In addition, for catchments where the elevation difference between the highest point of the headwater and sampling point (an approximate topographical parameter) exceeds 400 m, the R-squared value obtained from the temperature and SWR regression analysis is significantly increased, indicating that temperature is likely the most important factor controlling silicate weathering in the alpine region where a kinetic-limited regime dominates.

Sr isotopic characteristics and fractionation during weathering of a small granitic watershed system in the Jiuhua Mountains (eastern China)

To determine the influence of the Mesozoic Yanshanian orogeny in East Asia on contemporaneous Sr isotopic evolution of seawater, we systematically investigated the weathering profile, riverbed sediment and stream water in mono-lithological small granitic watersheds of the Jiuhua Mountains, Anhui, eastern China. Analysis based on 190 samples from 1 to 2 samplings per month during an entire hydrological year, spanning July 2014 to June 2015, shows that the intra-annual change of Sr concentration is 10–70%, but 87Sr/86Sr ratios only exhibit a slight change (0.709148–0.710427). This result indicates that using single sampling data to evaluate the influence of chemical weathering on the 87Sr/86Sr ratio evolution of seawater may cause some deviations. The 87Sr/86Sr ratio in the small granitic watersheds of the Jiuhua Mountains is 0.709148–0.710427 with an average of 0.710021, which is significantly higher than the lowest value (0.7068, ∼160 Ma) of seawater in the Phanerozoic. During and after this period, the East Asian continent experienced a strong tectonic event — Yanshanian orogeny and formed widespread Jurassic–Cretaceous igneous rocks, such as the Jiuhua Mountains granite in the Yangtze Block. The Yanshanian granites in several main tectonic units in China exhibit high radiogenic Sr characteristics. Combined with the evidence of enhanced chemical weathering during Late Jurassic-Early Cretaceous, the notable increase of the seawater 87Sr/86Sr ratio after 160 Ma may be related to the Yanshanian orogeny in East Asia.

Geochemical tracing and modeling of surface and deep water\u2013rock interactions in elementary granitic watersheds (Strengbach and Ringelbach CZOs, France)

From the study of the Strengbach and Ringelbach watersheds we propose to illustrate the interest of combining the geochemical tracing and geochemical modeling approaches on surface and deep borehole waters, to decipher the diversity of the water flow and the associated water–rock interactions in such elementary mountainous catchments. The results point to a clear geochemical typology of waters depending on the water circulations (deep vs. hypodermic) within the substratum.

Influence of pre-event water on streamflow in a granitic watershed using hydrograph separation

For water resources management, many studies for investigating flow paths from rainfall to subsurface have been conducted for last half century. A hydrograph separation based on end member mixing was carried out to evaluate the importance of the hydrological pathways providing the main sources of a small granitic watershed, Dorim-cheon, Seoul. An analysis of chloride, oxygen-18 and deuterium isotopes from precipitation and stream water during three storms was conducted with high-resolution data using 129 samples. Stream water, collected in advance of rain event, was assumed as a pre-event water (baseflow) component according to its dry condition and isotopic values compared with the values of different time periods. The contribution from vadose water was ignored due to the thin soil layer covering the study area. Using a two-component mixing model, hydrograph separation of the stream water in Dorim-cheon was conducted based on weighted mean values of rain water and pre-event water component with the high-resolution datasets. As a result of the analysis with water isotopes, contribution of groundwater was dominant during the entire study period (73–74%) except of instant dominance of rainfall at the earliest period. Using chloride as a tracer for hydrograph separation, a significant difference for the amount of pre-event water contribution was identified. This might be caused by the large variation of chloride concentration during the rain event and the end member determination.

Natural and Anthropocene fluxes of trace elements in estuarine sediments of Galician Rias

The Anthropocene has been proposed as a new geo-stratigraphic epoch where humans have become a global factor affecting the ecosystems. Estuarine sediments constitute a biogeochemical reservoir where trace elements (TEs) from natural sources mix with a fraction generated by human activities. This study assessed the natural lithogenic imprint of TEs from uncontaminated sedimentary records to study anthropogenic changes. Sediment cores were sampled in the estuarine part of the low populated and low industrialized rias of Ares (A), Betanzos (B) and Cedeira (C), which are located in the NW of the Iberian Peninsula. Cores were dated by the 210Pb method, covering up to the last century period. Sedimentation rates (in mm yr−1) were 3.4 ± 0.6 (older than 1961) and 8.2 ± 3.2 (younger than 1961) for core A, 0.9 ± 0.2 (<1940) and 4.2 ± 2.3 for core B, and 13.4 ± 1.4 for core C. The background fluxes of TEs were in the ranges of (in g m−2 yr−1) 50–560 for Al and Fe, 20–320 for POC and 8–100 for PS, and (in mg m−2 yr−1) 20–120 for As, 0.2–2.0 for Cd, 7–110 for Co, 40–870 for Cr, 20–230 for Cu, 0.1–0.6 for Hg, 290–2800 for Mn, 4–24 for Mo, 20–410 for Ni, 20–150 for Pb, 60–800 for V and 70–700 for Zn. The three cores showed an anthropogenic influence since the mid-20th century. The detected human impact during the Anthropocene was due to the Eume River damming and As-Pontes lignite mining (the largest mine open-pit in the Iberian Peninsula) in core A, by bridge/road constructions, vehicles emissions and river mouth urbanization in core B, and changes in land use in core C. Sediment accumulation rates increased 2.4 (core A) and 4.7 times (core B) while TEs fluxes increased 2.4 times in core A and between 3.6 and 5.1 times in core B. Pre-industrial sedimentary records showed the lithogenic differences of the drainage basins, mainly granitic watersheds for cores A and B and the prevalence of basic rocks of the Ortegal Geological Complex for core C. This explains the high concentrations and fluxes of Cr, Ni and V of core C. The different lithogenic imprints highlight the value of using local background references in contamination studies, such as those on the Anthropocene influence in estuaries, which occurred after the Spanish Civil War in the studied rias.

Mapping Lithology and Assessing Recharge Characteristics in a Granitic Hard Rock Aquifer: Inference from 2D Resistivity, Induced Polarization, Tracer and Moisture Measurements

Abstract: Two dimensional Electrical Resistivity Tomography (ERT) investigation along with Time Domain Induced Polarization (TDIP) investigation covering 1.6 km line were carried out at 3 natural recharge sites in a overexploited groundwater granite watershed, situated in a semi arid region in the state of Telangana, India. At these sites, shallow and/or deep moisture influx measurements were also carried out using injected tritium tracer and neutron moisture probe. The watershed is covered by sandy loam to silt loam soil, receives an average annual rainfall of 620 mm with the pre monsoon groundwater level ranging from 8m to 29m bgl. The spot investigations were done to assess and understand the recharge process and groundwater potential in terms of resistivity/conductivity and moisture characteristics of the subsurface rock formation. The measured and 2D inverted resistivity models shows soil, highly weathered and moderately weathered zones up to a depth of 22m followed by semi-weathered and massive granites up to the maximum depth of 78m. The resistivity values of the soil and shallow weathered zones varying between ~5-50 Ohm.m up to a depth of 12m. The large variation in resistivity values of the soil and weathered zone is probably indicative of large variation in the moisture potential. The resistivity models clearly mapped the granitic hard rock structure in the depth range of 22 – 78m, which has a significant resistivity contrast (~1500-11000 Ohm.m) with respect to the overlying semi weathered layers. The modeled 2D resistivity data at a site in two perpendicular directions shows similar geological structure and geoelectric layers. The TDIP dataset shows insignificant chargeability variations (1-7 mV/V) up to the investigated depth of 78m, which probably indicates low to moderate moisture and groundwater potential in the vadoze and saturated zones of the aquifer. The experimental investigations provide scope for assessing dynamic recharge and groundwater potential at selected sites in an overexploited granite watershed in the present geological context.

Using isotopes to determine the contribution of volcanic ash to Sr and Ca in stream waters and plants in a granite watershed, Mt. Tsukuba, central Japan

Strontium (Sr) isotope ratios of soils, stream water, plants, and atmospheric precipitation were used to trace calcium (Ca) and to determine the influence of volcanic ash in soils on Ca cycling in forest ecosystems in a small forested watershed on Mt. Tsukuba, central Japan. The 87Sr/86Sr ratios of soils on steep slopes and the valley floor ranged from 0.7110 to 0.7139, which indicates that the major source of Sr was the granite substrate (87Sr/86Sr ratio 0.7120–0.7131). Soils on a ridge had lower 87Sr/86Sr ratios (0.7068–0.7072), indicating that they were mainly composed of volcanic ash materials released around 30,000 years ago from Mt. Akagi (0.7069). The 87Sr/86Sr ratio decreased and the concentrations of Sr (and Ca) increased in stream water with increased elevation from the valley bottom to the ridge, indicating that volcanic ash was the dominant source of both cations in the upstream area. The contributions of volcanic ash-derived Sr to upstream and downstream water were 50 and 0–1 %, respectively. The 87Sr/86Sr ratios of plants were between those of the soils in which the plants were growing and those of atmospheric precipitation (0.7100). More than 74 % of the Sr in plants on the ridge, but <17 % in plants in the valley bottom, was volcanic ash-derived. The origin of Sr (and thus Ca) in stream waters and plants varied depending on the volcanic ash content in soils, which was significantly influenced by the site elevation.

Evaluating process domains in small arid granitic watersheds: Case study of Pima Wash, South Mountains, Sonoran Desert, USA

This paper provides support for the concept of geomorphic process domains developed by Montgomery (1999) by linking geomorphic processes to ecological variations seen in the Pima arid granitic watershed of the Sonoran Desert, Phoenix, Arizona. Closer joint spacing shows a statistically significant correlation with lower percentages of mineral grain attachment as measured by digital image processing of backscattered electron microscope imagery. Lower mineral grain attachment leads to more frequent spalling of rock surfaces, as measured by varnish microlamination (VML) ages of the last spalling event. In contrast, more distant joint spacing leads to in situ 10Be erosion rates of 3.4–8.5mm/ka and the emergence of low domes and kopje granitic landforms; these low domes also serve as knickpoints along ephemeral washes. Distant jointing thus plays a key role in generating the bare bedrock surfaces that funnel limited precipitation to bedrock margins — enhancing the canopy cover of perennial plants next to the bare bedrock. Joint-influenced geomorphic processes at Pima Wash generate four distinct process domains: (PD1) armored drainage divides; (PD2) slopes with different granite landforms; (PD3) mid- and upper basin channels that mix knickzones, strath floodplains, and sandy alluvial sections; and (PD4) the main ephemeral channel transitioning to the piedmont. Distant jointing promotes bedrock exposure and rock armoring along drainage divides in PD1 that then concentrates runoff and promotes perennial plant growth. More distant joint spacing on slopes in PD2 promotes exposure of granitic bedrock forms that shed overland flow to their margin and promotes flora and fauna growths along the margins of low granitic domes and kopjes. Similarly, wider joint spacing along ephemeral washes in PD3 leads to knickpoints, which in turn act to concentrate moisture immediately downstream. The stream terraces in PD4 influence the ecology through xeric desert pavements on terrace treads and roofs for coyotes (Canis latrans) and gray fox (Urocyon Cinereoargenteus) dens on terrace scarps via stage 3 pedogenic carbonate. These four process domains occur in six other randomly selected granitic watersheds with drainage areas <5km2 in the Mojave and Sonoran Deserts. Results on rates of geomorphic processes in the Pima Wash watershed provide new insight in the desert geomorphology of small granitic watersheds. Catchment-wide denudation rates (CWDRs) recorded by 10Be sampled along the main ephemeral wash vary between 15 and 23mm/ka and do not appear to be influenced by knickpoint or knickzone occurrence; instead slightly lower CWDRs appear to be associated with sediment contributions by subbasins with more abundance of bare bedrock forms. Resampling for CWDR after a 500-year flood event from hurricane moisture at two sites along the main ephemeral channel revealed no detectable changes; this finding confirms the average representativeness of CWDR as a long-term denudation proxy and also means that sediment transport on these arid granitic hillslopes must be incremental and without rapid crest to wash transport. The first reported measurements of incision rates into a small granitic Sonoran Desert watershed, using 10Be and VML, reveal rates on the order of 70–180mm/ka in the lower quarter of Pima Wash for the last 60ka — producing a narrow and deep trench. As this base-level fall propagates upstream, erosion focuses on weaker material with higher joint densities; this facilitates the emergence of domes and kopje landforms with more widely spaced jointing.

Old Water Contributions to a Granitic Watershed, Dorim-cheon, Seoul

It is reported that the intensity of rainfall will likely increase, on average, over the world on 2000. For water resources security, many studies for flow paths from rainfall or snowmelt to subsurface have been conducted. In Korea, few isotopic studies for characterizations of flow path have been undertaken. For a better understanding of how water derived from atmosphere moves to subsurface and from subsurface to stream, an analysis of precipitation and stream water using oxygen-18 and deuterium isotopes in a small watershed, Dorim-cheon, Seoul, was conducted with high resolution data. Variations of oxygen-18 in precipitation greater than 10‰ (δ 18 Omax = –1.21‰, δ 18 Omin = –11.23) were observed. Isotopic compositions of old water (groundwater) assumed as the stream water collected in advance were –8.98‰ and –61.85‰ for oxygen and hydrogen, respectively. Using a two-component mixing model, hydrograph separation of the stream water in Dorim-cheon was conducted based on weighted mean value of δ 18 O. As a result, except of instant dominance of rainfall, contribution of old water was dominant during the study period. On average, 71.3% of the old water and 28.7% of rainfall contributed to the stream water. The results show that even in the small watershed, which is covered with thin soil layer in granite mountain region, the stream water is considerably influenced by old water inflow rather than rainfall.

Time Lapse Tracer and SP Measurements to Characterize the Hydrodynamics of Fractured Granite Aquifer: A Case Study

Abstract: An attempt is made to identify and delineate the groundwater flow direction and rate in a fractured hard rock aquifer in Maheshwaram granite watershed in Andhra Pradesh using multiple tracers such as bromide, iodide and rhodamine-B under both natural and induced conditions. A main well in the center and three observation wells at 25 m in north-northwest, southeast and southwest directions respectively were constructed based on 222Rn anomalies and 4He measurements. Streaming potential survey was also carried out in the area before, during and after pumping test. The tracer tests indicated preferential groundwater flow in N-S direction. The groundwater flow velocity estimated based on arrival of tracer under natural and induced conditions are 0.52 m/d and 375 m/d respectively.

Field evidence of groundwater ridging in a slope of a granite watershed without the capillary fringe effect

Summary The controls on the rapid mobilization and runoff of pre-event water are an important issue in hillslope hydrology. One of the mechanisms involved is groundwater ridging, based on the capillary fringe effect. The typical conditions for groundwater ridging to occur are as follows: (1) the slope consists of fine- to medium-textured materials with a large extent of the tension-saturated zone, (2) the slope gradient is low, and (3) the initial groundwater level is high. Where these three conditions are met, a further condition, (4) a small amount of rainwater, would then be enough to trigger groundwater ridging. In this study, we detail groundwater ridge formation in the bedrock layers of Akakabe Watershed (Japan) as recorded by detailed field observations using tensiometers and bedrock boreholes under conditions violating the above. The study site consisted of materials showing almost no tension-saturated zone and had a relatively high gradient (22°). Moreover, the initially high groundwater level did not always generate a groundwater ridge, and a large total rainfall (>160 mm) was required to trigger groundwater ridging. Hence the conventional mechanism cannot explain the groundwater ridging at the field site studied here. It seems that the smaller distance from the groundwater table to the ground surface and the higher hydraulic conductivity in the downslope areas compared to the upslope areas caused a more rapid and larger groundwater level rise in the downslope areas, which accounted for the observed groundwater ridging. This study demonstrated that a groundwater ridge can be formed in the absence of the capillary fringe effect and that bedrock groundwater can contribute to storm runoff generation in headwater catchments under the influence of groundwater ridging.

Erosion and weathering fluxes in granitic basins: The example of the Strengbach catchment (Vosges massif, eastern France)

Abstract This work reports on regular observations of the suspended sediment and bedload flux at the outlet of the granitic Strengbach catchment (0.8 km²) over the period 2004–2010 along with the analysis of the dissolved load. The data set allows a first determination of the physical erosion flux (3.3 t·km − 2 ·yr − 1 ) exported out of the Strengbach watershed and how it compares to the chemical weathering flux of basic cations. The data indicate that the erosion rate carried by the bedload (1.7 t·km − 2 ·yr − 1 ) can reach one third of the total erosion rate (5 t·km − 2 ·yr − 1 ) in the Strengbach watershed. The accuracy of these rates is estimated to be 20%. The study also indicates that the estimated values for the net annual weathering flux calculated for the watershed can differ by a factor of two or more depending on the corrections applied and the assumptions retained for the watershed's functioning (from an uncorrected weathering flux of basic cations of 4.76 t·km − 2 ·yr − 1 to an atmosphere-corrected weathering flux of 2 t·km − 2 ·yr − 1 for the period 2004–2010). The weathering budgets determined for the Strengbach catchment fit the general trend defined between the weathering rate and runoff for other granitic watersheds; this suggests that the anthropogenic impact due to acid perturbation or forest management has a relatively weak influence on the weathering intensity within the watershed. The relationship between weathering and erosion rates determined for the Strengbach catchment does not significantly differ from those established for other previously studied granitoid catchments.

Combined geophysical methods for detecting soil thickness distribution on a weathered granitic hillslope

The usefulness of electrical resistivity imaging (ERI) as a highly accurate method for determining the soil thickness distribution on hillslopes was validated by combining intensive measurements using invasive methods, i.e., cone penetration testing and boreholes, with ERI in three granitic watersheds. Areas of high electrical resistivity (ρ) contrast reflecting soil–bedrock interfaces were found in all three study watersheds. However, ρ values of soil and weathered granite just below the soil mantle varied over a relatively wide range at each site, as well as considerably from site to site. The patterns of low–high contrast in ρ profiles, reflecting the soil–bedrock interface, also differed from site to site despite similarly dry conditions. Differences in the water retention characteristics of soil and weathered granitic bedrock, as found by a previous study of bedrock hydrological properties, may have been a major factor in the observed subsurface ρ variations. The ERI method, with electrode spacing of 0.5 to 2.0m, was successful in determining soil thickness distributions ranging from about 0.5 to 3m depth based on its ability to detect high contrast in ρ in the subsurface zone. Closer electrode spacings are expected to more sensitively reveal the distribution of ground material properties and thus more accurately replicate the soil–bedrock interface. ERI failed to clearly identify the soil–bedrock interface at some points along our measurement lines because of local intermediate materials with different properties such as unconsolidated soil and clayey intermediation just below the soil–bedrock interface. Two types of seismic survey (SS) techniques were also used, combining seismic refraction (SR) and the surface wave method (SWM) with the ERI method in a granitic watershed to compare ERI with other geophysical methods. The profile of S-wave velocity (Vs) by SWM also reasonably duplicated the soil–bedrock interface; the Vs profile showed larger variation in lateral direction and corresponded to the soil thickness distribution better than the P-wave velocity (Vp) profile by SR. The combined use of ERI and SWM may be more effective in detecting the soil–bedrock interface because each method compensates for the deficiencies of the other method.

Fluoride occurrence and mobilization in geo-environment of semi-arid Granite watershed in southern peninsular India

Plausible forms of fluoride (F) responsible for the persistence of fluoride toxicity in ground water of a granitic terrain of semi-arid region, which is the main source of drinking water, have been studied. The study area in Anantapur District of Andhra Pradesh, India, is one of the chronic regions with excess fluoride in groundwater and the region is under transformation into aridity due to poor rainfall and over-exploitation of groundwater. Geochemical analysis of soil, groundwater, and rock samples of the study area revealed the presence of other toxic elements also in addition to fluoride which need to be addressed in drinking water sector in near future. Soil fluoride leaching experiments demonstrated the probable mode of mobilization of F into the groundwater through natural recharge process during monsoon. Analysis of saturation indices indicates that the fluorite solubility alone is not attributable to the high fluoride content in groundwater. The groundwater flow controls fluoride mobilization in the study area as it is evidenced through fluoride concentration and electrical conductivity increase from catchment to downstream region. Creation of lesser fluoride groundwater sources through rainwater harvesting and artificial recharge of groundwater in catchment areas is suggested as a long-term sustainable safe drinking water strategy.

Sr fluxes and 87Sr/ 86Sr characterization of river waters from a karstic versus granitic watershed in the Yangtze River

The watershed in the southern Jiangxi Province (Jiangxi Province is called simply Gan) (SGW) and the watershed in the central Guizhou Province (Guizhou Province is called simply Qian) (CQW) are two subtropical watersheds of the Yangtze River in China. Both watersheds have similar latitudes and climate, but distinct differences in basin lithology. These similarities and differences provide a good natural laboratory in which to investigate weathering processes and Sr end-members in river waters. This work aims to identify and contrast the sources, fluxes and controls on Sr isotopic composition in the river waters of these two areas. Results showed that the 87Sr/ 86Sr in the SGW waters ranged from 0.716501 to 0.724931, with dissolved Sr averaging 27 μg l − 1 . Rhyolites and granites are two major sources for the dissolved Sr. The SGW waters receive 42% of their Sr from silicates weathering, 32% from carbonates and 3.2% from evaporites. 87Sr/ 86Sr in the CQW waters has a lesser variation from 0.707694 to 0.710039, but higher Sr contents (average of 208 μg l − 1 ). Dolomite, limestone and dolomitic limestone are major sources of Sr in the waters. The CQW waters receive 69% of their Sr from carbonates, 1.7% from silicates and 0.9% from evaporites. The chemical erosion rate and Sr flux in the CQW are 122 t km − 2 a − 1 and 0.079 t km − 2 a − 1 , respectively, which are higher than those of the SGW (56 t km − 2 a − 1 and 0.021 t km − 2 a − 1 , respectively). These data suggest that the intensive carbonates weathering occurred in the karstic area in the upper-reach of the Yangtze River exert great influence on the high Sr concentration and low Sr isotopic ratios in the River.

Chemical weathering and chemical runoffs in the seashore granite hills in South China

Surface runoffs and the contents of major ions and dissolved silicon were surveyed and analyzed based on weekly sampling for one year in a granite watershed of Zhuhai City, Guangdong Province. The results demonstrated that the slope runoff played an essential role in feeding the rivulet during precipitation, whereas the groundwater was more important for the rivulet in the non-raining stages. Dissolved silicon, Na+, HCO3 −, and Cl− ions constituted the major chemical components of the rivulet water. Atmospheric CO2 and H2SO4 sourced from the oxidation of pyrite were the main erosive mediums in the natural chemical weathering. Natural chemical weathering, the wet and dry deposition of sea-salt, and acids precipitation contributed approximately equal shares of anions to the rivulet water. The ratio of NO3 − to SO4 2− was more than 1.0 in the runoff of the studied seashore watershed, whereas it is less than 1.0 in the Xijiang River that is adjacent to the study area. This implied that the chemical composition of runoff is controlled mainly by the differences in atmospheric acid precipitation caused by human activities and lithology within the catchments under the same bio-climate zone. The CO2 flux consumed by the rock chemical weathering processes within the seashore granite watershed in South China was (0.35–1.37)×105 mol km−2 a−1.