Geochemical and Isotopic Assessment of Groundwater Quality in the Alluvial Aquifer of the Eastern Mitidja Plain

Water Resources - The Mitidja-plain is very important in size and role, it contains two important aquiferous tanks exploited to serve Algiers and the surrounding agglomerations, these resources...     

Hydrogeochemical Evolution Along Groundwater Flow Paths in the Manas River Basin,Northwest China

The impacts of long-term pumping on groundwater chemistry remain unclear in the Manas River Basin, Northwest China. In this study, major ions within five surface water and 105 groundwater samples were analyzed to identify hydrogeochemical processes affecting groundwater composition and evolution along the regional-scale groundwater flow paths using the multivariate techniques of hierarchical cluster analysis (HCA) and principal components analysis (PCA) and traditional graphical methods for analyzing groundwater geochemistry. HCA classified the groundwater samples into four clusters (C1 to C4). PCA reduced the dimensionality of geochemical data into three PCs, which explained 86% of the total variance. The results of HCA and PCA were used to identify three zones: “recharge,” “transition,” and “discharge.” In the recharge zone the groundwater type is Ca-HCO₃-SO₄ and is primarily impacted by the dissolution of calcite and silicate weathering. In the transition zone the groundwater type is Ca-HCO₃-SO₄-Cl and is impacted by rock dissolution and reverse ion exchange. In the discharge zone the groundwater type is Na-Cl and is impacted by evaporation and reverse ion exchange. In addition, anthropogenic activities impact the groundwater chemistry in the study area. The groundwater type generally changes from Ca-HCO₃-SO₄ in the recharge area to Na-Cl in the discharge area along the regional-scale groundwater flow paths. This study provides a process-based knowledge for understanding the interaction of groundwater flow patterns and geochemical evolution within the Manas River Basin.     

Unsaturated Zone Flow Processes and Aquifer Response Time in the Chalk Aquifer,Brighton, South East England

The Chalk aquifer is one of the main sources of water in South East England. The unsaturated zone in the aquifer plays an important role controlling the time and magnitude of recharge and is major pathway for contaminant transport to the water table. A range of previous work has addressed flow processes in the Chalk unsaturated zone, but physical understanding is still incomplete. Here we present the results of a study on flow mechanism in the Chalk unsaturated zone using a combination of statistical analysis and novel laboratory methods. The study was undertaken at three sites (North Heath Barn [NHB], Pyecombe East [PE], and Preston Park [PP]) on the Chalk of the Brighton block, South East England. Daily and hourly time series data of groundwater level and rainfall were correlated. The results show that a slower groundwater level response to rainfall occurs during dry seasons (summer and autumn) when the amount of effective rainfall is less than 4 mm/day, with a thicker and drier unsaturated zone. A faster response occurs during wet seasons (winter and spring) when the daily effective rainfall exceeds 4 mm/day with a thinner and wetter unsaturated zone. Periods of very rapid response (within 15 h) were observed during wet seasons at NHB and PE sites, with unsaturated hydraulic conductivity (K_u) inferred to reach 839 mm/day. A slower response was observed at an urbanized site (PP) as a result of reduction in direct recharge due to reduced infiltration, due to presences of impermeable infrastructure covering the area around PP borehole. Laboratory measurements of K_u of the Chalk matrix using a geotechnical centrifuge show variation from 4.27 to 0.07 mm/day, according to the level of saturation. Thus, the rapid responses cannot be linked to matrix flow only but indicate the contribution of fracture and karstic flow processes in conducting water.     

Estimating Local Ground-Water Flow Conditions in a Granitoid: Preliminary Assessments in the Waldoboro Pluton Complex, Maine

Periodic dry wells are reported by some homeowners in Knox and Lincoln Counties, Maine. These wells pump from the migmatitic Bucksport Fm. which is intermixed with the granitoids of the Waldoboro Pluton Complex. Previous remote sensing and geologic mapping delineated marked northeast-trending structures in the WPC which were initially suspected as influencing ground-water flow for sustainable well yields. A sample area, Pemaquid Pond, in the WPC was studied in more detail including preliminary hydraulic testing of homeowner wells and ground-waiter flow modeling. Wells with sustainable and greater yields appear to be associated instead with zones of northwest-trending structures in granitoids not intermixed with the Bucksport Fm. Dry wells appear to be more prevalent amongst northeast-trending structures in areas where the migmatitic metamorphic rocks outcrop. Hydraulic studies, including pumping, slug, pressure, and borehole dilution testing, resulted in a wide range of calculated hydraulic conductivities. Dual-porosity flow modeling and geostatistical parameterization of the flow conditions suggest that the anisotropic hydraulic conductivity is near E-9 m/d.