The detection of microbiological contamination in drinking water from groundwater wells is often made with a limited number of samples that are collected using traditional geochemical sampling protocols. The objective of this study is to examine the variability of fecal indicator bacteria, as observed using discrete samples, due to pumping. Two wells were instrumented as multilevel piezometers in a bedrock aquifer, and bacterial enumeration was conducted on a total of 166 samples (for total coliform, fecal coliform, Escherichia coli, and fecal streptococci) using standard membrane filtration methods. Five tests were conducted using pumping rates ranging from 0.3 to 17 L/min in a variety of purging scenarios, which included constant and variable (incremental increase and decrease) flow. The results clearly show a rapid and reproducible, 1 to 2 log-unit decrease in fecal indicator bacteria at the onset of pumping to stabilized, low-level concentrations prior to the removal of three to five well volumes. The pumping rate was not found to be correlated with the magnitude of observed bacterial counts. Based on the results, we suggest sampling protocols for fecal indicator bacteria that include multiple collections during the course of pumping, including early-time samples, and consider other techniques such as microscopic enumeration when assessing the source of bacteria from the well-aquifer system.

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This study focuses on assessing groundwater potability in a highly complex and heterogeneous fractured bedrock aquifer having variable overburden cover. Eight monitoring wells were installed in a privately serviced lakeside village, and groundwater was routinely sampled over a 2-year timeframe for concentration analysis of nitrate, fecal indicator bacteria, stable isotopes, and a total of 41 pharmaceutical compounds. While pollutant concentrations remained low throughout the study, the presence of fecal indicator bacteria and pharmaceuticals was noted at least once (but not always consistently) in most sampling intervals. An interpretation based on the integration of chemical, bacterial, and site characterization datasets suggests that: (1) the fracture network is complex and heterogeneous with limited vertical connectivity; (2) existing pathways are sufficient for the quick and widespread migration of surface contaminants to depth; (3) anthropogenic contaminants from both septic systems and agriculture are likely sourced in the surrounding uplands where overburden is thin; and (4) fecal contamination, as observed over the long term, is ubiquitous at the village scale. Groundwater quality is continually changing in this hydrogeologic environment and the determination of potability on the larger scale is not likely to be adequately captured with infrequent domestic well sampling (i.e., voluntary annual sampling by homeowners).

[1] In this study we examine the required spatiotemporal discretization for numerical simulation of field-scale radial solute transport using a discrete fracture model. Point-to-point and borehole-to-point scenarios considering a single fracture, hydrodynamic dispersion, and matrix diffusion were modeled in two-dimensional (2-D) and three-dimensional (3-D) domains, and were verified using existing semianalytical solutions. The results clearly show the need for accurate spatiotemporal discretization in numerical simulations of this type. Spatial discretization away from the injection point/well in the horizontal plane was found to be more demanding than away from the fracture in the vertical direction. Concentration control, which was used in the time step discretization of the transport solution, varied over two orders of magnitude depending on the scenario being modeled. The fine spatiotemporal discretization required may significantly limit the ability to simulate radial transport in more complex hydrogeological conditions (i.e., multiple fractures and injection points/wells) particularly in 3-D because of long computational runtimes.Citation: Kozuskanich, J. C., K. S. Novakowski, and B. C. Anderson (2012), Discretizing a discrete fracture model for simulation of radial transport, Water Resour. Res., 48, W03602,