Hydrogeochemical Evolution of an Aquifer Regulated by Pyrite Oxidation and Organic Sediments

Detailed full-scale groundwater monitoring was carried out over a period of nine years, sampling at selected points along the groundwater flow direction in its final stretch. This established the hydrogeochemical evolution along the flow of a natural system formed by a calcareous aquifer which discharges and then passes through a quaternary aquifer of lake origin which is rich in organic matter. This evolution is highly conditioned by the oxidation of pyrites that are abundant in both aquifers. In the first aquifer, one kilometre before the discharge location, oxidizing groundwater crosses a pyrite mineralization zone whose oxidation produces an important increase in sulphates and water denitrification over a short period of time. In the quaternary aquifer with peat sediments and pyrites, water experiences, over a small 500 m passage and residence time of between three and nine years, a complete reduction by way of pyrite oxidation, and a consequent increase in sulphates and the generation of hydrogen sulphuric acid. This is an example of an exceptional natural hydrogeological environment which provides guidance on hydrogeochemical processes such as denitrification.

Groundwater dynamics and groundwater surface-water exchange in the near-stream zone across the hydrologic year

<p>Groundwater dynamics and flow directions in the near-stream zone depend on groundwater gradients, are highly dynamic in space and time, and reflect the flowpaths between stream channel and groundwater. A wide variety of studies have addressed groundwater flow and changes of flow direction in the near-stream domain which, however, have obtained contrasting results on the drivers and hydrologic conditions of water exchange between stream channel and near-stream groundwater. Here, we investigate groundwater dynamics and flow direction in the stream corridor through a spatially dense groundwater monitoring network over a period of 18 months, addressing the following research questions:</p><ul><li>How and why does groundwater table response vary between precipitation events across different hydrological states in the near-stream domain?</li> <li>How and why does groundwater flow direction in the near-stream domain change across different hydrological conditions?</li> </ul><p>Our results show a large spatio-temporal variability in groundwater table dynamics. During the progression from dry to wet hydrologic conditions, we observe an increase in precipitation depths required to trigger groundwater response and an increase in the timing of groundwater response (i.e. the lag-time between the onset of a precipitation event and groundwater rise). This behaviour can be explained by the subsurface structure with solum, subsolum, and fractured bedrock showing decreasing storage capacity with depth. A Spearman rank (r<sub>s</sub>) correlation analysis reveals a lack of significant correlation between the observed minimum precipitation depth needed to trigger groundwater response with the local thickness of the subsurface layer, as well as with the distance from and the elevation above the stream channel. However, both the increase in groundwater level  and the timing of the groundwater response are positively correlated with the thickness of the solum and subsolum layers and with the distance and the elevation from the stream channel, but only during wet conditions. These results suggest that during wet conditions the spatial differences in the groundwater dynamics are mostly controlled by the regolith depth above the fractured bedrock. However, during dry conditions, local changes in the storage capacities of the fractured bedrock or the presence of preferential flowpaths in the fractured schist matrix could control the spatially heterogeneous timing of groundwater response. In the winter months, the groundwater flow direction points mostly toward the stream channel also many days after an event, suggesting that the groundwater flow from upslope locations controls the near-stream groundwater movement toward the stream channel during wet hydrologic conditions. However, during dry-out or long recessions, the groundwater table at the footslopes decreases to the stream level or below. In these conditions, the groundwater fall lines point toward the footslopes both in the summer and in the winter and in different sections of the stream reach. This study highlights the effect of different initial conditions, precipitation characteristics, streamflow, and potential water inflow from hillslopes on groundwater dynamics and groundwater surface-water exchange in the near stream domain.</p>

Assessment of river water–groundwater–seawater interactions in the coastal delta of Bangladesh based on hydrochemistry and salinity distribution

A synchronization study among hydrochemistry, hydrochemical facies evaluation, EC observation, salinity distribution and groundwater flow direction has been addressed to assess river water–groundwater–seawater interactions in the coastal delta of southern Bangladesh. The findings show that river water, shallow groundwater and deep groundwater interact with seawater at various intensities within the complex dynamics of hydrochemical facies evaluation. Deep groundwater is intensively influenced by seawater, where shallow groundwater is moderately affected and river water is very negligibly affected. Major cation and anion have been plotted in the Piper diagrams and hydrochemical facies diagrams (HFE-D) to clarify the result. More than 60% of the water samples of the river lie on the Ca-HCO3 (or Mg-HCO3) facies quadrant, and more than 70% of the shallow groundwater samples and more than 95% of the deep groundwater samples lie on the Na-Cl facies quadrant of the HFE-D diagram. River water types are dissimilar, and approximately 82% of facies are characterized by freshening phases and 18% by intrusion phases. Mixed water types with predominate of Na-Cl were observed in shallow groundwater where the hydrochemical facies are characterized by 53 percent freshening phases and 47 percent intrusion phases. Deep groundwater hydrochemistry clearly indicates the dominant Na-Cl type of water in the study area where only four hydrochemical facies are observed and 78 percent correspond to the intrusion phases and 22 percent to the freshening phases. Both direct and reverse cation exchange reactions take place in shallow groundwater, where deep groundwater is predominantly characterized by reverse cation exchange reactions. Two end members: seawater of Bay of Bengal and freshwater, contribute to the exchange reactions in the coastal aquifer of the study area. In terms of nitrate contamination, river waters are affected by negligible to low concentrations, shallow groundwater is affected by moderate to high concentrations and deep groundwater is affected by moderate to very high nitrate concentrations. Dissimilarity in electrical conductivity (EC) values, variation of salinity distribution maps and groundwater flow direction suggest the possible interconnections among river water, groundwater and aquifer sediments. Significant concentrations of Na+ and Cl− ions lead to seawater contamination in groundwater, and HCO3− along with Na+, Ca2+, Mg2+ in river water suggests mixing of freshwater and seawater, which could have adverse effects both in coastal delta aquatic life and in agriculture.

DELINEATION OF AQUIFEROUS ZONES USING GEOPHYSICAL METHODS AND CONTAMINANT FLOW DIRECTION -A case study of dumpsite at Cassidy, Okokomaiko, LASU-Badagry Expressway, Lagos State, Nigeria

Introduction: Cassidy area, of Okokomaiko, Ojo, Lagos State, is a rapidly developing area with attendant growing population arising from the presence of University, financial institutions and popularly known Alaba market. Unfortunately, water supply by the water corporation is not readily available. The inhabitants only rely on the surface water and groundwater extracted from hand dug wells and boreholes. Aims: The study is to map the aquiferous zone and establish the groundwater flow direction, with a view to averting subsurface contamination emanated from dumpsite within the area. Materials and Methods: Twenty five (25) Vertical Electrical Sounding (VES) using Schlumberger electrode array, four (4) Electrical Resistivity imaging using Wenner Electrode Array were acquired at four different traverses. Manual groundwater flow direction was also conducted. Results: The identified unconfined aquifer was sand/sandy clay which is overlain by peat/clay soil that is loose and may allow downward migration of the contaminant plumes to deep groundwater. However, the confined aquifer observed to occur approximately at depth 18 m. The electrical resistivity distribution observed decreases upward, south and north. This implies increase in conductivity in such directions and as such indicating the contaminant could be more concentrated near-surface, north and south. Conclusion: The implication of the results obtained with respect to vulnerability to groundwater pollution is that the southern section of the aquifer is more susceptible to receive transported contaminants from the northern part of the study area. The study suggests the potential borehole should be sited in the north-eastern direction with the aid of geophysical survey.

Influence of Subway Construction on Groundwater Environment in Downtown Area of Chengdu

<p>Located at the southeast of the Minjiang alluvial-proluvial fan, the downtown area of Chengdu mainly composed of sand gravel layer. Now Chengdu has 8 subway lines operated; in the next 10 years, more than 34 routes will be constructed. Metro Line 7 forming a transfer relationship with multiple urban MRTS and urban commuter radiation built completly in downtown area, with depth of subway station 1.73-11.3 m, and the depth of interval tunnel 6.47-28.01 m. In order to study how the groundwater will be influenced, 3 3d groundwater numerical models in different scales have been constructed using FeFlow software, the results illustrated regional groundwater seepage field and local seepage field.</p><p>Baed on 1 regional model (417 km2 for downtown Chengdu ) and 2 models of typical underground space (Taipingyuan station and Yipintianxia station), at the same time with the basic geology and hydrogeology Analysis, shows that:</p><p>(1) The influence of metro line 7 on the seepage field is relatively limited in regional scale, and the change of groundwater level is very little(4-10cm) at several typical observation points; in the long-term, the raising of groundwater level will decrease gradually.</p><p>(2) Comparing the simulation results of Taipingyuan station and Yipintianxia station shows the impact of subway construction on the groundwater environment in the downtown Chengdu. In the big view, from northwest to southeast, the phenomenon of underground water interception or raising in subway stations decrease gradually, this is owing to the influence of aquifer thickness, groundwater flow direction and the direction of underground station structure.</p><p>(3) As the main body or long section of the underground structure is coincide with the groundwater flow direction, the cross-section blocking the groundwater is minimized, so its influence on the groundwater seepage field is not notable even with development of the underground space, this is also help avoiding the floatation effect on the building foundation due to the raising of the groundwater flow.</p>

KONDISI SUMUR DAN PEMODELAN ARAH ALIRAN AIRTANAH BEBAS PADA BENTUKLAHAN FLUVIOMARIN DI JAKARTA

ABSTRACT Growing population have an impact on the strategy of fulfillment the water need and degradation of groundwater quality in Jakarta, especially in fluviomarine landforms in Jakarta. The purpose of this study was to determine the condition of wells and create a model of groundwater flow direction on fluviomarine landforms based on the season. Methods in this research study include three main aspects, namely population, characteristics of the object under study, and analysis. The population in this study using 30 groundwater wells sample representative of the population. Relating to the characteristics of the object under study, this study using a survey method. The survey is a sample survey on wells population who still use unconfined groundwater. About data analysis, then in this study used quantitative and qualitative approaches to the modeling of the groundwater flow direction using the Inverse Distance Weighted (IDW) in ArcGIS. The results showed that the unconfined groundwater wells in the study area consisted of dug wells and pantek wells. It is generally known that groundwater levels in the rainy season are higher than the dry season with a depth of groundwater level in the dry season to be deeper than the rainy season. Most of the groundwater level in organic settlements in the study area is below sea level, whereas in planned settlements is rarely found people who use groundwater. The depth of unconfined groundwater well less than 20 m with a thickness of water on the wells in the rainy season are thicker than the dry season. Groundwater flow direction along the north coast of central and western parts likely to lead to the mainland, while the southern part has a groundwater flow that is likely to lead to the North. Keywords: Fluviomarine Landforms In Jakarta; Unconfined Groundwater; Groundwater Flow Direction