No abstract

No abstract

Conceptual models of aquifer systems can be refined and complemented with geophysical data, and they can assist in understanding hydrogeological properties such as groundwater storage capacity. This research attempts to use geoelectrical methods, Electrical Resistivity Tomography and Induced Polarization parameters, for mapping the subsurface in alluvial fans and to demonstrate its applicability; the Punata alluvial fan was used as a case study. The resistivity measurements proved to be a good tool for mapping the subsurface in the fan, especially when used in combination with Induced Polarization parameters (i.e., Normalized Chargeability). The Punata alluvial fan characterization indicated that the top part of the subsurface is composed of boulders in a matrix of finer particles and that the grain size decreases with depth; the electrical resistivity of these deposits ranged from 200 to 1000 Ωm, while the values of normalized chargeability were lower than 0.05 mS/m. The bottom of the aquifer system consisted of a layer with high clay content, and the resistivity ranged from 10 to 100 Ωm, while the normalized chargeability is higher than 0.07 mS/m. With the integration of these results and lithological information, a refined conceptual model is proposed; this model gives a more detailed description of the local aquifer system. It can be concluded that geoelectrical methods are useful for mapping aquifer systems in alluvial fans.

The objective of the present study was to integrate the relative risk from mercury exposure to stream biota, groundwater, and humans in the Río Artiguas (Sucio) river basin, Nicaragua, where local gold mining occurs. A hazard quotient was used as a common exchange rate in probabilistic estimations of exposure and effects by means of Monte Carlo simulations. The endpoint for stream organisms was the lethal no-observed-effect concentration (NOECs), for groundwater the WHO guideline and the inhibitory Hg concentrations in bacteria (IC), and for humans the tolerable daily intake (TDI) and the benchmark dose level with an uncertainty factor of 10 (BMDLs(0.1)). Macroinvertebrates and fish in the contaminated river are faced with a higher risk to suffer from exposure to Hg than humans eating contaminated fish and bacteria living in the groundwater. The river sediment is the most hazardous source for the macroinvertebrates, and macroinvertebrates make up the highest risk for fish. The distribution of body concentrations of Hg in fish in the mining areas of the basin may exceed the distribution of endpoint values with close to 100% probability. Similarly, the Hg concentration in cord blood of humans feeding on fish from the river was predicted to exceed the BMDLs(0.1) with about 10% probability. Most of the risk to the groundwater quality is confined to the vicinity of the gold refining plants and along the river, with a probability of about 20% to exceed the guideline value.

A preliminary analysis of the groundwater recharge to the Karoo formations, midZambezi basin, ZimbabweLarsen, F; Owen, R; Dahlin, Torleif; Mangeya, P; Barmen, Gerhard Link to publication Citation for published version (APA): Larsen, F., Owen, R., Dahlin, T., Mangeya, P., & Barmen, G. (2002). A preliminary analysis of the groundwater recharge to the Karoo formations, mid-Zambezi basin, Zimbabwe. Physics and Chemistry of the Earth, 27(11-22), 765-772. DOI: 10.1016 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal AbstractA multi-disciplinary study is being carried out on recharge to the Karoo sandstone aquifer in the western part of Zimbabwe, where recharge is controlled by the presence of a thick, confining basalt layer. The aquifer is geographically extensive, and has been identified throughout the southern part of the mid-Zambezi basin (Fig. 1). The potential for groundwater abstraction seems to be huge.The key issues in this part of the study are the extent of the recharge area and the recharge rates. The direct recharge area has previously been considered to be the area of outcrop of Karoo Forest sandstone, before it dips below an impervious basalt cover. However, resistivity profiling shows that the basalt at the basin margin is weathered and fractured, and probably permeable, while the basalt deeper into the basin is fresh, solid and impermeable. Field and laboratory analysis of 22 groundwater samples support this extension of the recharge area to include a large area below the fractured basalt. CO 2 gas pressures, calculated with the code PHREEQC using field measurements of pH and alkalinity, show that below the fractured basalt the groundwater is an open system in contact with atmospheric CO 2 . The 14 C and nitrate concentrations in this groundwater also indicate that recent infiltration takes place.The chloride contents of the rainfall and the groundwater in the recharge area have been measured to calculate direct recharge from rainfall. These data indicate that the direct recharge is in the range of 10-130 mm/yr, with an average value of 25 mm/yr. Preliminary results of recharge estimate using 36 Cl data suggests lower direct infiltration rates, but further studies are needed.The combination of hydro-chemical, isotopic and geophysical investigations show that the recharge area extends well beyond the sandstone outcrop area, northwards beneath the basalt some 20 km beyond the basalt margin.