Development of Novel Tracer Techniques for Better Understanding of Karst Transport Characteristics and Surface Water Influences

Abstract

The primary concern for surface water influenced systems is to limit the risk associated with Giardia cysts and Cryptosporidium oocysts. Published research indicates that Cryptosporidium and Giardia can survive for months in some aquatic environments, are resistant to chlorination, and may cause infection in small numbers. Karst aquifers are generally expected to be the more susceptible to this type of contamination than granular aquifers, because of the rapid flow velocities and lower potential for filtration in these systems. However, even in karst aquifers, flow rates and recharge characteristics can vary greatly, so not all wells or springs in these settings may be considered as groundwater under the direct influence (GWUDI) of surface water. The purpose of this research is to develop and test alternative methods for assessing GWUDI in karst aquifers using advanced microbial (PCR-based molecular) and isotopic techniques. The study will include a comparison of the efficacy of these methods with current EPA protocols and other common indicators of microbial contamination. These new techniques will also be evaluated for consideration in describing the transport characteristics of microbial organisms in a karst subsurface environment. This is an on-going study in the very early stages. This paper will present the experimental design and conceptualization. Introduction In 1989, the U.S. Environmental Protection Agency promulgated the Surface Water Treatment Rule (SWTR), as 40 CFR Part 141, Subpart H, to require that public water supplies derived from, “ground water under the direct influence of surface water” (GWUDI), receive the same degree of treatment as surface water sources (ie. filtration and disinfection). The primary concern for these designated water systems is to limit the risk associated with Giardia cysts and Cryptosporidium oocysts. Published research indicates that Cryptosporidium and Giardia can survive for months in some aquatic environments, are resistant to chlorination, and may cause infection in small numbers (Robertson et al. 1992; Heisz 1997; Craun 1990; DuPont et al. 1995). Karst aquifers are generally expected to be the more susceptible to this type of contamination than granular aquifers, because of the rapid flow velocities and lower potential for filtration in these systems. However, even in karst aquifers, flow rates and recharge characteristics can vary greatly, so not all wells or springs in these settings may be considered as GWUDI. Copyright ASCE 2005 EWRI 2005 Because the costs associated with treating GWUDI are substantially higher than the costs related to treating “true groundwater” (ie. any groundwater which isn’t classified as GWUDI), there is a need to determine whether existing methods are appropriate and whether they can be supplemented by other, more efficient and defensible methods. The purpose of this proposal is to develop and test alternative methods for assessing GWUDI in karst aquifers using advanced microbial (PCR-based molecular) and isotopic techniques. The study will include comparing the efficacy of these methods with current EPA protocols and other common indicators of microbial contamination. Background The U.S. EPA currently defines GWUDI as any subsurface water that (1) has a significant occurrence of insects or other microorganisms, alga, organic debris, or largediameter pathogens (i.e. Giardia lamblia), or (2) significant and relatively rapid changes in water characteristics such as turbidity, temperature, conductivity, or pH which can be correlated to climatological or surface-water conditions (Chin and Qi 2000). The U.S. EPA developed the microscopic particulate analysis (MPA) for determining if existing ground water resource should be classified under the GWUDI classification (USEPA 1992). The MPA requires the filtering of 500 to 1,000 gallons of pumped water and conducting a microscopic analysis of the filtrate. The filtrate material is compared with the occurrence or relative concentration of indicators defined in Table 1 (Chin and Qi 2000; Hoffbuhr et al. 1986). An empirical estimation of the relative risk of surface water contamination is made by comparing the classification under Table 1 to the relative risk factors in Table 2. In Tennessee, determination of whether or not a system is classified as GWUDI is typically made by staff in the field offices. Samples are usually collected after a “major” rain event, but because of the dynamic nature of karst flow systems, the results of a given MPA test can vary according to the magnitude of the rain event, antecedent soil moisture conditions, and other seasonal factors. As well, some of the MPA indicators may be lost from the water due to filtration or gravitational settling, even in situations where pathogens may be rapidly transported. As a result, it is preferable to have a range of different indicators for assessing GWUDI and to develop empirical relationships between occurrence of different types of microbial, chemical and physical indicators. Copyright ASCE 2005 EWRI 2005 Table 1 . Classification Protocol from Relative Concentration of Primary Bio-Indicators