Perched Groundwater Research Articles

Geohydrology Surrounding a Potential High-Level Nuclear Waste Repository in the Paradox Basin, Utah

ABSTRACTThe Gibson Dome area in the Paradox Basin in southeastern Utah (Figure 1) has been identified as a potential location for a high-level nuclear waste repository on the basis of an adequate thickness of bedded salt formations at desirable depths, suitable topography for surface facilities, few known archaeological sites, less resource potential than otherwise similar areas, and long-term geologic and tectonic stability [1]. The area appears also suitable from a geohydrologic viewpoint, on the basis of data collected and analyses performed to date. The regional geohydrologi information was obtained from three deep boreholes drilled as part of the Department of Energy's exploration program and from other existing sources. The upper, near-surface, geologic formations include both regionally continuous water-bearing formations and locally perched ground waters that discharge into nearby surface streams and into the Colorado River. These are underlain by the Paradox formation, which consists of a series of essentially impermeable salt layers and of nonsalt interbeds with negligible ground water. Below the Paradox salts, the formations of interest with respect to repository safety include regionally continuous water-bearing formations, with the Leadville limestone being the principal water-transmitting unit. Flows in all water-bearing formations are essentially horizontal. If a vertical connection were established through a potential repository, hydraulic gradients indicate that the flow would first be downward from the upper to the lower formations and then laterally, principally in the Leadville formation. Surface discharge of Leadville formation ground water from the Gibson Dome area has not been detected. There are some indications that minor leakage could occur into the Colorado River as close as Cataract Canyon, about 20 to 25 km (10 to 15 miles) from a potential repository location in Davis Canyon, or into the Colorado River in Marble Canyon (Arizona), about 240 km (150 miles) to the southwest. The computed ground-water flow time in the Leadville formation from below Davis Canyon is about 90,000 years for the first 10 km (6 miles) and about 200,000 years to below Cataract Canyon. The ground-water flow time is estimated to exceed 500,000 years to Marble Canyon. Groundwater flow from a repository to these locations is unlikely, however, since water would first have to penetrate the essentially impermeable salt layers before it would reach the Leadville formation.

A case study of a chemical spill: Polychlorinated biphenyls (PCBs): 2. Hydrogeological conditions and contaminant migration

The purpose of the second paper in this pair is to describe the hydrogeology and to account for the observed patterns of PCB contamination at the Federal Pioneer site in Regina, Canada. Six major hydrostratigraphic units were identified from detailed test drilling and sampling. From the surface downward these are a thin surface fill, the Regina clay, the Condie silt, the till zone, the interglacial silt, and the Regina aquifer system. This latter unit is an important source of potable groundwater. At present the nearest wells are hundreds of meters from the site. The permanent water table is situated below the Condie silt in sandy glacial till. An active perched groundwater system has developed within the thin fill layer beneath the floor of the plant. The extensive lateral flow of groundwater in this zone has contributed significantly to the spreading of PCBs at the site. The PCBs have also moved downward through fractures in the Regina clay and in the upper part of the Condie silt. It can be expected that capillary forces will halt the downward movement of Inerteen 70‐30® as fractures narrow or stop. However, the possible existence of fractures connecting the contaminated zone to the Regina aquifer system or the potential for transport as a dissolved phase in the groundwaters may permit PCBs migrate downward to the aquifer or to flow laterally off‐site. The most significant hazard represented by the spill is the potential for contamination of future water supply wells that may be placed near the site or aquifer contamination that could result if future drilling provides pathways for contaminants through poorly sealed wells or test holes.

Movement of heavy metals into a shallow aquifer by leakage from sewage oxidation ponds

The concentrations of Mn, Ni, Cu, Cd and Cr were measured in a shallow perched groundwater aquifer which underlies the Dan Region Sewage Reclamation Project (Israel). The contribution of effluents to the groundwaters has been evaluated on the basis of chloride concentration. Groundwater which are estimated to contain more than 60% effluents showed a hundred-fold decrease in Cu and Mn at a distance of 650 m away from the ponds, as compared with the near ponds samples. Ni and Cd showed only a small decrease in concentration over a distance of 150 m, and then stayed constant. The concentrations of Cu and particularly of Mn in the groundwaters near the oxidation ponds is equivalent to or greater than in the ponds themselves. It is suggested that Cu and Mn are mobilized from the precipitated sludge into the interstitial waters. They percolate into the groundwater near the ponds and then are precipitated by increasing aeration during the movement of the water away from the pond area. Cd and Ni form stable soluble organic chelates which are only slightly removed by interaction with the sandy soil of the aquifer.

Effects of power line construction upon the carbonate water chemistry of part of a mid-latitude swamp

Summary Beverly Swamp in southern Ontario is typical of a wetland in carbonate terrain. At two sites in the swamp, a study was conducted between 1975 and 1977 to determine the impact of a power-line construction programme upon the carbonate water chemistry of the perched ground water. Construction activities included deforestation along a narrow corridor across the swamp as well as drilling, pumping and grouting during the erection of towers. The latter resulted in (1) contamination of the perched ground water by the alkaline water forced to the surface from near the dolomite bedrock, and (2) disturbance of the peat and marl structure. These in turn induced higher pH, total hardness and bicarbonate concentrations in the perched ground water. Although such effects persisted beyond the construction period, the spatial patterns exhibited by the pertinent hydrochemical variables indicate that the impacts were spatially confined. It is inferred that recurrent flooding will enhance recovery of the disturbed sites.

Shapes of steady state perched groundwater mounds

A potential theory flow solution for the potential function, stream function, and shape of the water table is given for a class of steady state two‐ and three‐dimensional perched groundwater mounds formed under a long rectangular recharge basin or under a circular recharge basin. The solutions are done by a Gram‐Schmidt method and a simple iteration scheme. The mounds are formed in a stratum of conductivity k1 overlying a perching stratum of much lower conductivity k2. Capillary fringe effects are neglected. The recharge rate is R. Potential theory mound heights are compared with those given by the Dupuit‐Forchheimer (DF) theory. For the cases computed (R/k2 = 10, 100, and 500 and k1/k2 = 50,100, 500, 600, 750, and 1000) the DF theory gives apex heights of mounds correct to better than 7% for two‐ dimensional mounds. For three‐dimensional mounds the DF theory gives in one case a mound height that is 69% too low (for R/k2 = 500 and k1/k2 = 600) and in another case a mound height that is 28% too low (for R/k2 = 500 and k1/k2 = 1000). Profiles of the computed mounds are graphed, and examples of use of the graphs in applications are given. Sample flow nets are presented.

Phosphorus Distribution from Septic Tank Effluent in Coastal Plain Soils

AbstractPhosphorus (P) accumulations in two soils with high perched water tables were monitored as a function of distance and depth to determine the fate of P from septic effluent in natural soil systems. The soils studied were Varina and Goldsboro sandy loams. Piezometers were placed at selected distances and depths from the drainfield in the direction of ground‐water flow to determine both vertical and lateral P movement. Soil P was fractionated to determine the type and extent of soil‐P fractions adjacent to the drainfield. Orthophosphate (PO4−3‐P) concentrations in the perched ground waters decreased with distance from the septic tank drainfield in both the Varina and Goldsboro soils. PO4−3‐P concentrations at the Varina soil location decreased from an average of 5.5 µg/ml in the undiluted effluent to 0.32 µg/ml in the soil solution at 3.0 m distance. No PO4−3‐P was detected in waters percolating through a very slowly permeable underlying plinthic horizon. PO4−3‐P concentrations in the Goldsboro soil solutions decreased from an average of 11.8 µg/ml in the drainline to <0.20 µg/ml at 3.0 m distance. In the Goldsboro soil P was concentrated in the 142 to 152 cm depth. Soil P fractions in the drainfield areas were present predominantly as NH4F and NaOH extractable P with some organic‐P present adjacent to the drainfield. At the Varina location, P accumulated in the argillic horizons. The NH4F and NaOH extractable P fractions increased from 2–4 and 40–50 µ/g in the control profile to a maximum of 460 and 425 µg/g in the argillic horizons at 0.15 m, respectively. At the Goldsboro location, septic effluent had not appreciably altered the quantities of “fixed” P or the distribution of soil P fractions.

Influence of Soils on Bacterial Contamination of a Watershed from Septic Sources

AbstractSurface and perched ground waters collected from an 80‐ha watershed were evaluated for bacteriological quality by enumeration of total and fecal coliform densities. This watershed is located at the western edge of the Coastal Plain Physiographic Province and the eastern edge of the Piedmont Plateau. Soils present have developed from both Piedmont and Coastal Plain materials. Analyses of surface waters from areas influenced by failing septic systems demonstrated a large range in total and fecal coliform densities. Sites not subjected to runoff from areas that contained failing septic systems had no detectable fecal coliform count in four of nine observations, whereas, in other sites where septic effluent was coming directly to the surface without passing through any soil material, the total and fecal coliform densities were in excess of 2.4 × 107/100 ml at some dates. Most of the surface waters in this watershed would not meet minimum recommendations for surface waters to be used for primary contact sports or public water supplies. Total and fecal coliform counts for subsurface samples generally decreased as a function of distance from the source of pollution.

Isotopic and chemical characteristics of high‐level groundwaters on Oahu, Hawaii

High‐level dike‐confined groundwaters and high‐level perched groundwaters on Oahu, Hawaii, were analyzed for tritium, radiocarbon, carbon 13, and several chemical constituents. Discharges from several tunnels that tap dike compartments contain postbomb tritium, indicating the presence of modern (since 1954) recharge. The amount of postbomb tritium in samples taken inside one of the tunnels is a function of the distance between sampling point and overlying surface. Chemical and carbon isotopic compositions of dike waters, established in the soil zone of the overlying ground surface, remain unchanged while the water is passing through the basaltic carbonate‐free compartments. Dike water carbon isotope data are sufficiently consistent to serve as initial values for the dating of water in the Honolulu basal aquifers. The radiocarbon and carbon 13 data for perched waters are similar to those for the dike waters, in spite of the much larger bicarbonate concentrations in the former.

The Study of Local Waters in the Deserts of the USSR

AbstractA review of potential local fresh-water sources in the deserts for the use of livestock herds, herders' settlements, and extractive and other industrial enterprises. One of the most promising approaches is held to be the use of takyrs [clay pans] as catchment areas for ephemeral runoff of precipitation, which is then directed into natural aquifers or artificial underground storage cisterns. Another potential source of water is large perched ground-water lenses that could be tapped by paired wells to prevent the intrusion of salt water from below. Artesian waters in the desert which are far from areas of recharge are found to be largely saline.