Reinhard K. Frohlich

Use of geoelectrical methods in groundwater pollution surveys in a coastal environment

Abstract The pollution of coastal aquifers by old landfills can contaminate valuable and scarce water resources in the freshwater lens utilized seasonably by overcrowded communities. The pollutants will ultimately flow into the sea where they may also cause a coastal water pollution problem. We have detected pollution in the freshwater lens from a sanitary landfill near Provincetown, Cape Cod, using the geoelectrical resistivity method. This survey included Schlumberger geoelectrical depth soundings and a horizontal geoelectrical profile using the Wenner configuration. The geoelectrical survey was conducted at a site along Highway 6 where it passes the coastal town of Provincetown and a sanitary landfill that has been in operation since 1954. The depth soundings suggest the characteristic decrease in resistivity vs. depth from the high resistivity of the unsaturated zone to the low resistivity of the saltwater saturated zone. The freshwater lens is clearly identified by the change in slope of the steeply dipping curve of resistivity versus electrode spacing. Interpretations made using a multilayer program, Geomate , resulted in layer resistivities between 460 and 95 ohm·m for the freshwater lens. A comparison with well water resistivities suggests that a layer resistivity of 230 ohm·m or lower is indicative of pollution in the freshwater lens. The results of the geoelectrical depth soundings were confirmed in the Wenner horizontal profile. Both measurements suggest that the pollutants do not spread evenly as one would expect for a homogeneous and isotropic medium. Instead, a preferred channel for the flow of the pollutants is observed along a path from the landfill toward the shoreline. The depth to the saltwater/freshwater interface or, more specifically, to the low resistivity-high resistivity interface appears to be shallow where the freshwater lens is polluted. This was confirmed by pore water well samples that were highly mineralized. The equilibrium postulated by the Ghyben-Herzberg relation appears to be disturbed in the area of aquifer pollution. This rise in the conductivity boundary is caused by the highly mineralized bottom of the contaminant plume that submerges into the saltwater saturated zone. In the area of high freshwater pollution the groundwater can be subdivided into three layers that show a decrease in resistivity with depth. The formation factor, F , defined as the ratio of bulk aquifer resistivity to pore water resistivity, shows unusually high values between 10 and 12. These high values are unexpected for an unconsolidated sand. Pollution residues are suspected to clog the pores and thus to increase the resistivity. It is possible that iron-oxidizing bacteria and the precipitation of dissolved iron or organic pollutants are the cause of the high values of F . If proven correct, these interesting possibilities could lead to future new applications of the geoelectrical resistivity method in contaminant hydroloy.

Surface electrical resistivity in coastal groundwater exploration

Abstract Because of its potential to detect changes in pore-water salinity the surface electrical resistivity method can be a valuable aid in coastal groundwater exploration and investigations. It is essential, however, that the resistivity interpretation be consistent with a hydrogeological model reflecting the fresh-water-salt-water relationship of coastal aquifers. In the electrical resistivity interpretation of phreatic aquifers it should be recognized that the lower boundary of the unsaturated zone corresponds to the top of the capillary zone, not to the water table, and that the lower boundary of the fresh-water layer corresponds only approximately to the top of the fresh-water-salt-water transition zone. The existence of a fresh-water layer can be ascertained qualitatively by visual inspection of electrical sounding curves, provided there is a fresh-water/unsaturated layer thickness ratio of at least four. Good interpretative methodology using an appropriate coastal hydrogeological model can enable the extent of the fresh-water layer to be quantified, but it is not possible to quantify the thickness of the transition zone by geoelectrical interpretation because of suppression effects.

Electric-hydraulic conductivity correlation in fractured crystalline bedrock: Central Landfill, Rhode Island, USA

Remote sensing and geoelectrical methods were used to find water-bearing fractures in the Scituate granite under the Central Landfill of Rhode Island. These studies were necessary to evaluate the integrity of the sanitary landfill and for planning safe landfill extensions. The most useful results were obtained with fracture trace analysis using Landsat and SLAR imagery in combination with ground-based resistivity measurements using Schlumberger vertical electrical soundings based on the assumption of horizontally layered strata. Test borings and packer tests confirmed, in the presence of a lineament and low bedrock resistivity, the probable existence of high bedrock fracture density and high average hydraulic conductivity. However, not every lineament was found to be associated with high fracture density and high hydraulic conductivity. Lineaments alone are not a reliable basis for characterising a landfill site as being affected by fractured bedrock. Horizontal fractures were found in borings located away from lineaments. High values of hydraulic conductivity were correlated with low bedrock resistivities. Bedrock resistivities between 60 and 700 Ω m were associated with average hydraulic conductivities between 4 and 60 cm/day. In some cases very low resistivities were confined to the upper part of the bedrock where the hydraulic conductivity was very large. These types of fractures apparently become narrower in aperture with depth. Bedrock zones having resistivities greater than 1000 Ω m showed, without exception, no flow to the test wells. Plots of bedrock resistivity versus the average hydraulic conductivity indicate that the resistivity decreases with increasing hydraulic conductivity. This relationship is inverse to that found in most unconsolidated sediments and is useful for estimating the hydraulic conductivity in groundwater surveys in fractured bedrock. In appropriate settings such as the Central Landfill site in New England, this electric-hydraulic correlation relationship, supplemented by lineament trace analysis, can be used effectively to estimate the hydraulic conductivity in bedrock from only a limited number of resistivity depth soundings and test wells.

The relation between hydraulic transmissivity and transverse resistance in a complicated aquifer of glacial outwash deposits

Abstract The application of the electrical resistivity method to freshwater aquifers has shown a direct relationship between formation factor and aquifer permeability. This is possible if the layer resistivity of the saturated zone can be obtained from an interpretation of the geoelectrical sounding curve without the ambiguity due to nonuniqueness. This is usually the case for uncomplicated horizontal layer cases where the aquifer is sufficiently thick and has a resistivity which can easily be distinguished from the resistivities of the unsaturated zone and the acquitard. In this study geoelectrical depth soundings after Schlumberger were conducted over aquifers in glacial outwash deposits near a recessional moraine of Rhode Island in southern New England. Lateral as well as vertical changes in lithology, in particular of the grainsize distribution, present a complicated system of a multistory aquifer. The depth soundings were interpreted in terms of layer resistivities, thicknesses, and transverse resistances. In most cases the aquifer could be identified with an intermediate resistivity between that of the unsaturated zone and the fine-grained aquitard. Results of the depth soundings were compared with test wells and pumping tests. Comparison of the ranges of hydraulic transmissivities with the transverse resistance shows a linear relationship between both values. This suggests that even in complicated cases, where test wells are of limited use, the geoelectrical depth sounding method can be used to locate aquifers of high hydraulic transmissivity. Great care is necessary to measure reliably the water resistivity in the test well. A further uncertainty is the depth of the filter screen relative to the bottom of the aquifer in light of the transitory nature between aquifer and aquitard. The transverse resistance is a more unique integral electrical parameter than the layer resistivity. This may explain why it shows a more reliable relationship with the hydraulic transmissivity in a complicated case than the relationship between formation factor and hydraulic conductivity, which was found in simple cases.

The use of geoelectrics and test wells for the assessment of groundwater quality of a coastal industrial site

Abstract Using geoelectrical depth soundings, potential groundwater pollution was assessed on a former shipyard of the Newport Naval Base, Rhode Island, USA. Besides an increase of the total dissolved solids (TDS), toxic organic compounds and metals were expected in the saturated zone. Two nested test wells were used to support the geoelectrical interpretation and a third one was located in a poorly accessible place. Non-invasive and cost-effective geoelectrics is representative for a sizeable volume, but the field data are non-unique with respect to the interpretation. Test wells, on the other hand, provide precise data, but they are laterally confined, so that a combination of both methods optimizes the results. This survey concentrates on the freshwater-saturated zone of glacial sediments that range in thickness between 1 and 15 m. The sediment is a poorly sorted till that is described as silty, gravelly sand with a low hydraulic conductivity of considerable heterogeneity. The major effort concentrated on a 12-m wide strip located between a sheet pile bulkhead along the shore and a building in which chemicals were stored and handled. This strip was the only place where depth soundings with a depth penetration extending below the freshwater-saturated zone were logistically possible. Other places were unsuitable because of conductive constructions at the surface, concrete pavements, and adjacent buildings. In spite of poor data quality, four of the five geoelectrical depth soundings were identified to be of QH-type ( ρ 1 > ρ 2 > ρ 3 ρ 4 ), with the following hydrogeological correspondence: unsaturated-, freshwater-saturated, seawater-saturated zone, and non-conductive tight glacial till and shale bedrock. The target of this investigation is the freshwater-saturated zone, which is qualitatively recognized on four of the five sounding curves. To avoid an underestimation of the interpreted layer resistivities, the lateral effect from the seawater and iron sheet pile bulkhead was corrected. It was approximated with a semi-infinite vertical boundary between the aquifer and a conducting medium using the image method. Low aquifer resistivities are usually associated with potential pollution in terms of high concentrations of TDS. A semi-quantitative attempt is described to correlate bulk layer with pore water resistivity and finally with equivalent TDS. Because of the presence of clay-sized particles in the sediment, the bulk electrical conductivity is caused by water conduction through the pore channels and the surface conductivity of the particles. Lab sample measurements of resistivity on samples recovered from the test wells provided a calibration curve for the bulk conductivity–water conductivity relationship. In spite of complications due to surface constructions and the heterogeneity of the freshwater-saturated till, it was possible to identify high conductivity groundwater with a conductivity on the order of 3000 μS/cm that is equivalent to approximately 1950 ppm TDS. It is not possible to determine whether or how much of the TDS originates from seawater overwash and how much is attributable to shipyard activities. Test wells, however, showed objectionable amounts of dissolved metals and adsorbed toxic organic compounds.

Estimates of specific yield with the geoelectric resistivity method in glacial aquifers

Abstract Specific yield is the volume of water that drains from a saturated soil due to gravity relative to its total volume. Methods to determine this hydrogeologic parameter depend on well tests and well sample analyses. Low-cost surface geoelectrical depth sounding after Schlumberger is suggested as an alternative method to estimate specific yield. This method is based on a determination of the layer resistivities for the saturated and unsaturated zone and on the water resistivity. Porosity and the degree of saturation are estimated from the electrical resistivity according to Archies Law. Specific yields estimated with the suggested geophysical method agree with estimates after Preuss and Todd which are based on porosity and median grain size data. Uncertainties in the relationship between resistivities and porosity and saturation exist due to generalized assumptions on material constants in Archies Law. For the range of specific yield estimates between 18 and 36% the constants seem to be valid. The method after Preuss and Todd requires a detailed evaluation of porosity and median grain size; it produces best results for D 50 values that are larger than 0.3mm. These values can vary considerably vertically as well as horizontally in the glacial aquifers derived mainly from fluvial outwash of the receding glaciers in southern New England.