Assessing the Vulnerability of a Municipal Well Field to Contamination in a Karst Aquifer

Abstract

Proposed expansion of extractive lime-rock mines near the Miami-Dade County Northwest well field and Everglades wetland areas has garnered intense scrutiny by government, public, environmental stakeholders, and the media because of concern that mining will increase the risk of pathogen contamination. Rock mines are excavated to the same depth as the well field's primary producing zone. The underlying karst Biscayne aquifer is a triple-porosity system characterized by (1) a matrix of interparticle porosity and separate vug porosity; (2) touching-vug porosity that forms preferred, stratiform passageways; and, less commonly, (3) conduit porosity formed by thin solution pipes, bedding-plane vugs, and cavernous vugs. Existing ground-water flow and particle tracking models do not provide adequate information regarding the ability of the aquifer to limit the advective movement of pathogens and other contaminants. Chemical transport and colloidal mobility properties have been delineated using conservative and microsphere-surrogate tracers for Cryptosporidium parvum. Forced-gradient tests were executed by introducing conservative tracers into injection wells located 100 m (328 ft) from a municipal-supply well. Apparent mean advective velocity between the wells is one to two orders of magnitude greater than previously measured. Touching-vug, stratiform flow zones are efficient pathways for tracer movement at the well field. The effective porosity for a continuum model between the point of injection and tracer recovery ranges from 2 to 4 percent and is an order of magnitude smaller than previously assumed. Existing well-field protection zones were established using porosity estimates based on specific yield. The effective, or kinematic, porosity of a Biscayne aquifer continuum model is lower than the total porosity, because high velocities occur along preferential flow paths that result in faster times of travel than can be represented with the ground-water flow equation. Tracer tests indicate that the relative ease of contaminant movement to municipal supply wells is much greater than previously considered.

Use of particulate surrogates for assessing microbial mobility in subsurface ecosystems

“…Following such an approach is particularly useful since properties of many tracers and their interactions with the environment cannot always be inferred from the literature. However, the authors are aware of only one study where microsphere tracer employed to karst groundwater is compared to the response of a real microorganism in laboratory tests (Renken et al 2005).…”

“…Several authors have identified the benefits of employing particulate tracers instead of solutes for assessing the risk of particulate contamination impacting groundwater quality in a range of hydrogeological environments (including karst), where fluorescent microspheres have been widely used as surrogates for bacteria (Harvey et al 1989;Harvey and Harms 2002;Auckenthaler et al 2002) and protozoa (Renken et al 2005) of comparable size. However, although particle size is frequently the dominant criterion considered in microsphere selection, conclusive evidence of their suitability has rarely been demonstrated.…”

Mitigation Banks and Other Conservation Lands at Risk from Preferential Groundwater Flow and Hydroperiod Alterations by Existing and Proposed Northeast Florida Mines

“…For example, Renken et al (2005) used tracers to calibrate models used in south Florida by regulatory agencies to evaluate the impacts of groundwater flow and potential contamination of municipal supply wells from additional mining proposed in the vicinity of those supply wells. Tracers used in that calibration even revealed groundwater flow occurring in the vicinity of these mine pits that was orders of magnitude greater than predicted by the models developed by the regulatory agencies.…”

CO2 Leakage-Induced Contamination in Shallow Potable Aquifer and Associated Health Risk Assessment

“…In order to understand such processes, the 2D cross-sectional potable aquifer model was designed as shown in Figure 3. The potable aquifer was assumed to be relatively deep (300 m with a reducing condition), where the municipal or high-capacity wells produced a large amount of groundwater, which was then distributed for residential purposes [48,49]. In addition, the width and thickness of the potable aquifer were assigned as 200 m and 40 m, respectively, with the size of grid blocks of 2 m; the total number of grid blocks was 2,000 (100 × 20).…”