Arsenic Management Through Well Modification and Simulation

Arsenic concentrations can be managed with a relatively simple strategy of grouting instead of completely destroying a selected interval of well. The strategy of selective grouting was investigated in Antelope Valley, California, where groundwater supplies most of the water demand. Naturally occurring arsenic typically exceeds concentrations of 10 µg/L in the water produced from these long-screened wells. The vertical distributions of arsenic concentrations in intervals of the aquifer contributing water to selected supply wells were characterized with depth-dependent water-quality sampling and flow logs. Arsenic primarily entered the lower half of the wells where lacustrine clay deposits and a deeper aquifer occurred. Five wells were modified by grouting from below the top of the lacustrine clay deposits to the bottom of the well, which reduced produced arsenic concentrations to less than 2 µg/L in four of the five wells. Long-term viability of well modification and reduction of specific capacity was assessed for well 4-54 with AnalyzeHOLE, which creates and uses axisymmetric, radial MODFLOW models. Two radial models were calibrated to observed borehole flows, drawdowns, and transmissivity by estimating hydraulic-conductivity values in the aquifer system and gravel packs of the original and modified wells. Lithology also constrained hydraulic-conductivity estimates as regularization observations. Well encrustations caused as much as 2 µg/L increase in simulated arsenic concentration by reducing the contribution of flow from the aquifer system above the lacustrine clay deposits. Simulated arsenic concentrations in the modified well remained less than 3 µg/L over a 20-year period.     

Electromagnetic‐Induction Logging to Monitor Changing Chloride Concentrations

Water from the San Joaquin Delta, having chloride concentrations up to 3590 mg/L, has intruded fresh water aquifers underlying Stockton, California. Changes in chloride concentrations at depth within these aquifers were evaluated using sequential electromagnetic (EM) induction logs collected during 2004 through 2007 at seven multiple‐well sites as deep as 268 m. Sequential EM logging is useful for identifying changes in groundwater quality through polyvinyl chloride‐cased wells in intervals not screened by wells. These unscreened intervals represent more than 90% of the aquifer at the sites studied. Sequential EM logging suggested degrading groundwater quality in numerous thin intervals, typically between 1 and 7 m in thickness, especially in the northern part of the study area. Some of these intervals were unscreened by wells, and would not have been identified by traditional groundwater sample collection. Sequential logging also identified intervals with improving water quality—possibly due to groundwater management practices that have limited pumping and promoted artificial recharge. EM resistivity was correlated with chloride concentrations in sampled wells and in water from core material. Natural gamma log data were used to account for the effect of aquifer lithology on EM resistivity. Results of this study show that a sequential EM logging is useful for identifying and monitoring the movement of high‐chloride water, having lower salinities and chloride concentrations than sea water, in aquifer intervals not screened by wells, and that increases in chloride in water from wells in the area are consistent with high‐chloride water originating from the San Joaquin Delta rather than from the underlying saline aquifer.     

Application of Temperature and Electrical-Conductivity Logging in Ground Water Monitoring

Down-hole temperature and electrical-conductivity probes are attractive logging devices for use in monitoring wells, because they are readily available, relatively inexpensive, and allow for rapid measurements. Benefits of such logging are illustrated by examples from several small sites in complex hydrogeologic settings. Interpretation of the logs provided information on the effectiveness of well purging, aquifer heterogeneities and the rates of ground water movement, leaks and cross-flows within wells, and locations of transmissive fractures in bedrock wells. Thus the logs offered an inexpensive means of acquiring valuable field information to supplement the geologic, potentiometric and chemical data collected during construction or subsequent investigation of the wells.     

Applying the HPT‐GWS for Hydrostratigraphy,Water Quality and Aquifer Recharge Investigations

The primary objective of this study was to evaluate use of the hydraulic profiling tool‐groundwater sampler (HPT‐GWS) log data as an indicator of water quality (level of dissolved ionic species) in an alluvial aquifer. The HPT‐GWS probe is designed for direct push advancement into unconsolidated formations. The system provides both injection pressure logs and electrical conductivity (EC) logs, and groundwater may be sampled at multiple depths as the probe is advanced (profiling). The combination of these three capabilities in one probe has not previously been available. During field work it was observed that when HPT corrected pressure (P_c) indicates a consistent aquifer unit then bulk formation EC can be used as an indicator of water quality. A high correlation coefficient (R ² = 0.93) was observed between groundwater specific conductance and bulk formation EC in the sands and gravels of the alluvial aquifer studied. These results indicate that groundwater specific conductance is exerting a controlling influence on the bulk formation EC of the coarse‐grained unit at this site, and probably many similar sites, consistent with Archie's Law. This simple relationship enables the use of the EC and P_c logs, with targeted water samples and a minimum of core samples, to rapidly assess groundwater quality over extended areas at high vertical resolution. This method was used to identify both a brine impacted zone at the base of the aquifer investigated and a groundwater recharge lens developing below storm water holding ponds in the upper portion of the same aquifer. Sample results for trace level, naturally occurring elements (As, Ba, U) further demonstrate the use of this system to sample for low level groundwater contamination.     

Management of the Wellfield Contaminated by Dense Chlorinated Hydrocarbons in Vilnius,Lithuania

Drinking water supply in Lithuania is entirely based on groundwater. Wellfields of Vilnius develop intermorainic ca. 50 m deep (in average) aquifer which is locally contaminated by chlorinated hydrocarbons— volatile organic compounds (VOCs). Groundwater abstraction activates VOCs migration from an abandoned factory into the pumping wells of one wellfield named “Vingis.” However monitoring data testify that only traces of VOCs were detected on the territory of this factory. Subsequent studies revealed the “secret”: dense VOCs have migrated from the territory of the polluter and have accumulated in lowermost places of pumped aquifer inside the wellfield. An attempt to ensure low concentration of VOCs in pumped water manipulating by pumping rates of more or less contaminated abstraction wells was not effective. Finally, an acceptable concentration of VOCs in supplied drinking water was ensured by permanent pumping out of the most polluted groundwater from some abstraction wells of the wellfield and diverting this water to the Neris River.     

Vertical Hydraulic Gradient Estimation in Clay Till,Using MiHPT Advanced Direct-Push Technology

The vertical transport of contaminants from source areas is employed in many risk assessment models and screening tools in order to estimate the contaminant mass discharge (CMD) into underlying aquifers. The key parameters for estimating CMD are the contaminant source area and concentration, and the vertical water flux, the latter of which depends on the vertical hydraulic conductivity and the vertical hydraulic gradient in the subsurface. This study focuses on advancing the use of the combined membrane interface probe hydraulic profiling tool (MiHPT) to investigate the vertical hydraulic gradient across a clay till overlying a sandy aquifer at a contaminated site in Denmark. Only the HPT is necessary for the estimate of vertical hydraulic gradient. The hydraulic head, clay till thickness, and resulting vertical hydraulic gradients found using the MiHPT compared well with observations from nearby nested wells. The parameter with the largest discrepancy was the thickness of the clay till. The advantage of the MiHPT is its relatively quick depth discrete access to information regarding subsurface permeability, vertical hydraulic gradients and contaminant distribution across a site. In this case study, performance of additional dissipationtests during the HPT log to acquire determination of the vertical hydraulic gradient increased the cost by 3% compared to standard HPT logs.     

Geophysical monitoring of a field-scale biostimulation pilot project

The USGS conducted a geophysical investigation in support of a U.S. Naval Facilities Engineering Command, Southern Division field-scale biostimulation pilot project at Anoka County Riverfront Park (ACP), down-gradient of the Naval Industrial Reserve Ordnance Plant, Fridley, Minnesota. The goal of the pilot project is to evaluate subsurface injection of vegetable oil emulsion (VOE) to stimulate microbial degradation of chlorinated hydrocarbons. To monitor the emplacement and movement of the VOE and changes in water chemistry resulting from VOE dissolution and/or enhanced biological activity, the USGS acquired cross-hole radar zero-offset profiles, travel-time tomograms, and borehole geophysical logs during five site visits over 1.5 years. Analysis of pre- and postinjection data sets using petrophysical models developed to estimate VOE saturation and changes in total dissolved solids provides insights into the spatial and temporal distribution of VOE and ground water with altered chemistry. Radar slowness-difference tomograms and zero-offset slowness profiles indicate that the VOE remained close to the injection wells, whereas radar attenuation profiles and electromagnetic induction logs indicate that bulk electrical conductivity increased down-gradient of the injection zone, diagnostic of changing water chemistry. Geophysical logs indicate that some screened intervals were located above or below zones of elevated dissolved solids; hence, the geophysical data provide a broader context for interpretation of water samples and evaluation of the biostimulation effort. Our results include (1) demonstration of field and data analysis methods for geophysical monitoring of VOE biostimulation and (2) site-specific insights into the spatial and temporal distributions of VOE at the ACP.     

Types and Usages of Drilling Fluids Utilized to Install Monitoring Wells Associated with Metals and Radionuclide Ground Water Studies

Evaluation for metal and radionuclide contamination associated with selected inactive uranium mill tailings repositories required the installation of monitoring wells in both shallow unconfined and deep artesian aquifer systems. To prevent ionic exchange between the dissolved cationic metals and the cations in bentonitic drilling fluids, organic-based fluids were used during the drilling and installation of monitoring wells. Experience gained at three western U.S. sites involving inactive uranium mill tailings indicates organic drilling fluids can be used to advance drill holes in soil materials ranging from saturated silts and fine sands to unsaturated gravels. However, it has been determined that certain types of these organic fluids can clog very narrow screen slots and remain present in well discharge, even after several hours of well cleaning. Certain types of organic drilling fluids appear to be preferable for these types of water chemistry studies.     

History of the Sole Source Aquifer Program: A Community-Based Approach for Protecting Aquifers Used for Drinking Water Supply

The Sole Source Aquifer Program has helped prevent contamination of many community drinking water supplies. If an aquifer supplies the sole or principal source of a community's drinking water, a local ground water user may petition the Environmental Protection Agency (EPA) under the Safe Drinking Water Act for its designation and protection as a "sole source aquifer." Since 1974, residents and officials of 65 communities and multi-community areas have petitioned and received assistance from the EPA to prevent contamination of their local ground water source of drinking water. This designation means that EPA may review federal financially assisted projects to determine if they would contaminate the aquifer and cause a public health hazard. If they could cause contamination, EPA can request that the project be modified or stopped. The significance of this program in terms of population served and funds affected has been substantial, indicating the Sole Source Aquifer Program has been an important local tool for protecting ground water used as a source of drinking water. Information is given on three different examples of sole source aquifer designations protected under this program: the New Jersey Coastal Plain Aquifer System, the Great Miami River Buried Valley Aquifer System (Ohio), and the Eastern Snake River Plain Aquifer (Idaho), serving populations of 543,000, 921,000, and 275,000, respectively. In all three examples, preventing ground water contamination through the Sole Source Aquifer Program has protected the community drinking water supply.     

Correlation between nitrate contamination and ground water pollution potential

AQUIPRO, a PC-based method, was used to assess aquifer vulnerability using digital water well logs. The AQUIPRO model is a parameter/factor weighting system for rating the pollution potential of an aquifer. This method uses the well depth, as well as the clay and partial clay thickness in a well, to generate pollution potential scores. In this model, aquifer protection increases as the AQUIPRO vulnerability scores increase and ground water pollution potential decreases. Computerized water well records of 2435 domestic wells with partial chemistry data were used to determine the ground water pollution potential of Kalamazoo County, Michigan. Theoretically, low AQUIPRO pollution potential scores should have more frequent occurrences of ground water contamination events than areas with high AQUIPRO scores with similar land-use, well construction, and well densities. The relative AQUIPRO scores were compared with the frequency of occurrences of nitrate-N in ground water wells. The average nitrate-N concentrations within each relative AQUIPRO vulnerability scores category were also compared. The results indicate that domestic wells containing 5 mg/L or more nitrate-N showed a positive correlation between the frequency of occurrences of nitrate-N and relative decrease of AQUIPRO (r2 = 0.99) vulnerability scores. In other words, as the ground water pollution potential increases, the occurrence frequency of nitrate-N also increases. Furthermore, the results show that as the relative AQUIPRO (r2 = 0.96) vulnerability scores decrease, the mean nitrate-N concentrations also increase.     

On using simple time-of-travel capture zone delineation methods

As part of its Wellhead Protection Program, the U.S. EPA mandates the delineation of "time-of-travel capture zones" as the basis for the definition of wellhead protection zones surrounding drinking water production wells. Depending on circumstances the capture zones may be determined using methods that range from simply drawing a circle around the well to sophisticated ground water flow and transport modeling. The simpler methods are attractive when faced with the delineation of hundreds or thousands of capture zones for small public drinking water supply wells. On the other hand, a circular capture zone may not be adequate in the presence of an ambient ground water flow regime. A dimensionless time-of-travel parameter T is used to determine when calculated fixed-radius capture zones can be used for drinking water production wells. The parameter incorporates aquifer properties, the magnitude of the ambient ground water flow field, and the travel time criterion for the time-of-travel capture zone. In the absence of interfering flow features, three different simple capture zones can be used depending on the value of T . A modified calculated fixed-radius capture zone proves protective when T < 0.1, while a more elongated capture zone must be used when T > 1. For values of T between 0.1 and 1, a circular capture zone can be used that is eccentric with respect to the well. Finally, calculating T allows for a quick assessment of the validity of circular capture zones without redoing the delineation with a computer model.     

Analysis of steady ground water flow toward wells in a confined-unconfined aquifer

A confined aquifer may become unconfined near the pumping wells when the water level falls below the confining unit in the case where the pumping rate is great and the excess hydraulic head over the top of the aquifer is small. Girinskii's potential function is applied to analyze the steady ground water flow induced by pumping wells with a constant-head boundary in a mixed confined-unconfined aquifer. The solution of the single-well problem is derived, and the critical radial distance at which the flow changes from confined to unconfined condition is obtained. Using image wells and the superposition method, an analytic solution is presented to study steady ground water flow induced by a group of pumping wells in an aquifer bounded by a river with constant head. A dimensionless function is introduced to determine whether a water table condition exists or not near the pumping wells. An example with three pumping wells is used to demonstrate the patterns of potentiometric surface and development of water table around the wells.     

Hydrostratigraphic modeling of a complex,glacial-drift aquifer system for importation into MODFLOW

Deposition from at least three episodes of glaciation left a complex glacial-drift aquifer system in central Illinois. The deepest and largest of these aquifers, the Sankoty-Mahomet Aquifer, occupies the lower part of a buried bedrock valley and supplies water to communities throughout central Illinois. Thin, discontinuous aquifers are present within glacial drift overlying the Sankoty-Mahomet Aquifer. This study was commissioned by local governments to identify possible areas where a regional water supply could be obtained from the aquifer with minimal adverse impacts on existing users. Geologic information from more than 2,200 existing water well logs was supplemented with new data from 28 test borings, water level measurements in 430 wells, and 35 km of surface geophysical profiles. A three-dimensional (3-D) hydrostratigraphic model was developed using a contouring software package, a geographic information system (GIS), and the 3-D geologic modeling package, EarthVision. The hydrostratigraphy of the glacial-drift sequence was depicted as seven uneven and discontinuous layers, which could be viewed from an infinite number of horizontal and vertical slices and as solid models of any layer. Several iterations were required before the 3-D model presented a reasonable depiction of the aquifer system. Layers from the resultant hydrostratigraphic model were imported into MODFLOW, where they were modified into continuous layers. This approach of developing a 3-D hydrostratigraphic model can be applied to other areas where complex aquifer systems are to be modeled and is also useful in helping lay audiences visualize aquifer systems.     

Hydrologic trade-offs in conjunctive use management

An aquifer, in a stream/aquifer system, acts as a storage reservoir for groundwater. Groundwater pumping creates stream depletion that recharges the aquifer. As wells in the aquifer are moved away from the stream, the aquifer acts to filter out annual fluctuations in pumping; with distance the stream depletion tends to become equal to the total pumping averaged as an annual rate, with only a small fluctuation. This is true for both a single well and an ensemble of wells. A typical growing season in much of the western United States is 3 to 4 months. An ensemble of irrigation wells spread more or less uniformly across an aquifer several miles wide, pumping during the growing season, will deplete the stream by approximately one-third of the total amount of water pumped during the growing season. The remaining two-thirds of stream depletion occurs outside the growing season. Furthermore, it takes more than a decade of pumping for an ensemble of wells to reach a steady-state condition in which the impact on the stream is the same in succeeding years. After a decade or more of pumping, the depletion is nearly constant through the year, with only a small seasonal fluctuation: ±10%. Conversely, stream depletion following shutting down the pumping from an ensemble of wells takes more than a decade to fully recover from the prior pumping. Effectively managing a conjunctive groundwater and surface water system requires integrating the entire system into a single management institution with a long-term outlook.     

Groundwater chemistry in intrapermafrost taliks in Central Yakutia

An intrapermafrost aquifer system, which is widespread in the sand deposits of bestyakhskaya terrace of the Lena R. (Central Yakutia), is characterized by generalized data of many-year studies of its groundwater chemistry. The groundwater discharges onto land surface through high-yield springs. The largest such source forms the Ulakhan-Taryn Creek with a mean many-year yield of 20 740 m³/day. The results of generalization were used to show the chemistry of intrapermafrost water to be stable at both many-year and annual scales, to characterize the hydraulic interaction of intrapermafrost water with suprapermafrost and subpermafrost water, to assess the spatial variations of groundwater resources in the intrapermafrost aquifer from the head-formation to the discharge domain. The results of the study are of importance for solving the problem of centralized drinking water supply to large populated localities in the Central Yakutia, including Yakutsk City.     

Gamma ray spectrometric analysis of hydrothermally altered dacite samples from the PACMANUS hydrothermal field: Implications for the interpretation of gamma ray wireline measurements

Spectral gamma ray wireline logging derives the concentrations of potassium, thorium, and uranium of the formation by measuring the gamma ray spectrum under the assumption of secular equilibrium. These measurements were carried out in a number of boreholes drilled by Ocean Drilling Program Leg 193 at the PACMANUS hydrothermal field. Spectral gamma ray logs from several of these holes show intervals of increased radioactivity, associated with high uranium values of up to 60 ppm (Snowcap hydrothermal field) and 25 ppm (Roman Ruins hydrothermal field). Nine samples of hydrothermally altered dacite were analyzed using gamma spectrometry to examine the origin of elevated radioactivity and to test for distortion of secular equilibrium. Core spectrometry indicates that secular equilibrium is distorted. Distortion can be explained by either an uptake of nuclides of the lower part of the ²³⁸U decay series, or by removal of ²³⁸U or ²³⁴Th from the rocks. In all cases wireline uranium logs and core spectrometry do not reflect true uranium concentrations of the formation but uranium is overestimated by a factor of 3–4. At Roman Ruins, uranium values from wireline logging show higher uranium concentrations over large intervals than uranium values from core spectrometry and from published core geochemistry, even when a possible leaching of uranium is taken into account. Wireline logs indicate that depth intervals of increased radioactivity are related to the occurrence of stockwork mineralization. In these depth intervals, core recovery was extremely low. In addition to the distortion of secular equilibrium, low core recovery and preferential loss of softer and more altered material cause a sampling bias between core samples and wireline data and may explain the observed difference in uranium concentrations between core and wireline logs.     

Detection of Contaminant Plumes by Borehole Geophysical Logging

Two borehole geophysical methods—electromagnetic induction and natural gamma radiation logs—were used to vertically delineate landfill leachate plumes in a glacial aquifer. Geophysical logs of monitoring wells near two land-fills in a glacial aquifer in west-central Vermont show that borehole geophysical methods can aid in interpretation of geologic logs and placement of monitoring well screens to sample landfill leachate plumes.
Zones of high electrical conductance were delineated from the electromagnetic log in wells near two landfills. Some of these zones were found to correlate with silt and clay units on the basis of drilling and gamma logs. Monitoring wells were screened specifically in zones of high electrical conductivity that did not correlate to a silt or clay unit. Zones of high electrical conductivity that did not correlate to a silt or clay unit were caused by the presence of ground water with a high specific conductance, generally from 1000 to 2370 μS/cm (microsiemens per centimeter at 25 degrees Celsius). Ambient ground water in the study area has a specific conductance of approximately 200 to 400 μS/cm. Landfill leachate plumes were found to be approximately 5 to 20 feet thick and to be near the water table surface.     

Geometry and dynamics of the freshwater—seawater interface in a coastal aquifer in southeastern Spain

Abstract

The contact between freshwater and seawater in coastal aquifers is studied using a relatively simple model for homogeneous aquifers. However, for real aquifers it is not so simple. The desalination plant built to supply water to the city of Almería is situated over the aquifer in the southern part of the River Andarax Delta. Its design capacity is 1100 L s^?1, and it is supplied from boreholes pumping water from beneath the freshwater—seawater contact in this aquifer. Well logs kept over a period of two years have allowed us to accurately define the interface geometry of the freshwater—seawater contact. Lithological data collected from 31 boreholes have also indicated the existence of strata with low hydraulic conductivity, within others of high conductivity. During a simultaneous pumping test of six wells with 690 L s^?1 total discharge, electrical conductivity measurements showed the influx of seawater 6–10 m below sea level and a drawdown of the interface in the piezometers closest to the pumping wells.     

Geostatistics applied to cross-well reflection seismic for imaging carbonate aquifers

Cross-well seismic reflection data, acquired from a carbonate aquifer at Port Mayaca test site near the eastern boundary of Lake Okeechobee in Martin County, Florida, are used to delineate flow units in the region intercepted by two wells. The interwell impedance determined by inversion from the seismic reflection data allows us to visualize the major boundaries between the hydraulic units. The hydraulic (flow) unit properties are based on the integration of well logs and the carbonate structure, which consists of isolated vuggy carbonate units and interconnected vug systems within the carbonate matrix. The vuggy and matrix porosity logs based on Formation Micro-Imager (FMI) data provide information about highly permeable conduits at well locations. The integration of the inverted impedance and well logs using geostatistics helps us to assess the resolution of the cross-well seismic method for detecting conduits and to determine whether these conduits are continuous or discontinuous between wells. A productive water zone of the aquifer outlined by the well logs was selected for analysis and interpretation. The ELAN (Elemental Log Analysis) porosity from two wells was selected as primary data and the reflection seismic-based impedance as secondary data. The direct and cross variograms along the vertical wells capture nested structures associated with periodic carbonate units, which correspond to connected flow units between the wells. Alternatively, the horizontal variogram of impedance (secondary data) provides scale lengths that correspond to irregular boundary shapes of flow units. The ELAN porosity image obtained by cokriging exhibits three similar flow units at different depths. These units are thin conduits developed in the first well and, at about the middle of the interwell separation region, these conduits connect to thicker flow units that are intercepted by the second well. In addition, a high impedance zone (low porosity) at a depth of about 275 m, after being converted to ELAN porosity, is characterized as a more confined low porosity structure. This continuous zone corresponds to a permeability barrier in the carbonate aquifer that separates the three connected conduits observed in the cokriging image. In the zones above and below this permeability barrier, the water production is very high, which agrees with water well observations at the Port Mayaca aquifer.     

Efficacy of Annual Bacteria Monitoring and Shock Chlorination in Wells Finished in a Floodplain Aquifer

Public health authorities generally recommend annual water-quality monitoring of rural water wells and shock chlorination if coliforms are detected. It is implicitly assumed that shock chlorination is effective in ridding most wells of bacteriological pathogens for months to years. Neither annual monitoring nor shock chlorination was effective in addressing coliform contamination of selected water wells in a small town developed on an alluvial aquifer where septic system effluents are impacting well water quality. Considerable temporal variation in total and fecal coliforms was observed in water wells monitored for a six-month period. Individual wells intermittently met and exceeded the drinking water criteria, indicating annual sampling was insufficient. Shock chlorination of three contaminated wells and their associated distribution systems proved ineffective because colonies apparently originated from outside the wells and reappeared over relatively short time periods (ranging from less than one week up to 21 weeks). The relatively fast and similar rate of recovery of total heterotrophic bacteria suggested they are related to biofilm formation in the wells and not to ground water contamination.