GROUND WATER/SURFACE WATER INTERACTIONS AWRA SUMMER SPEC~TY CONFERENCE 2002

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1 JULY 1-3 GROUND WATER/SURFACE WATER INTERACTIONS AWRA SUMMER SPEC~TY CONFERENCE 2002 HYDROGEOLOGIC ASSESSMENT OF GROUNDWATER UNDER DIRECT INFLUENCE OF SURFACE WATER Mark Fulkerson and Fidelia N. Nnadi ABSTRACT: Crypfosporidium parvum is a waterborne pathogen known to reside in surface waters throughout the US. Recent Cryptosporidiosis outbreaks from drinking water wells are the result of this contaminant transporting between surface and ground water systems. With a high resistance to typical groundwater treatment procedures, aquifer infiltration by Crypfosporidium poses a serious threat. Since groundwater wells are the main source of drinking water supply in the State of Florida, understanding certain factors that affect the passage of C. parvum into the subsurface would prevent future outbreaks. This study examines Florida s surficial geology, recharge capacity, and possible sinkhole development as they influence the risk of groundwater contamination. Microscopic Particulate Analysis (MPA) sampling was performed on 719 public water systems distributed throughout Florida. Sampling results defined the wells as having high, moderate, or low risk to surface water influence. The results of this study indicated that the hydrogeology of an area tends to influence the MPA Risk Index (RI) of a well. Areas of higher aquifer recharge generally contained the majority of the moderate and highrisk wells. Surficial geologic regions of shelly sand and clay, limestone, and clayey sand contained over 80% of the high-risk wells. The results also suggested that areas prone to sinkhole development are likely to contain wells with positive RI. KEY TERMS: Cryptosporidium parvum; groundwater contamination; karst region; hydrogeology; microscopic particulate analysis; Safe Drinking Water Act. INTRODUCTION Drinking water is quite certainly the single most importance resource for human existence. In the State of Florida, over 90% of the water used for public supply comes from aquifer systems. Generally thought to be of higher bacteriological quality than surface waters, treatment of these groundwater sources is commonly limited to disinfection and filtration. In recent years, however, it has been shown that certain contaminants (Le. C. parvum) are passing through these conventional treatments and entering distribution systems. Due to the increased number of outbreaks and subsequent illness, Crypfosporidium parvum has emerged as one of the most significant waterborne pathogens (Haas et al ). This protozoan parasite, ranging in size from 3 to 7 microns (Nnadi and Sharek, 1999), carries a high resistance to typical chlorination processes and is not deterred by common filtration systems. It can survive for several months outside its host, encased in a hardshelled oocyst (Medema et al., 1997). The American Water Works Association, in a study conducted from 1988 to 1993, found the presence of C. parvum in over 60% of the surface water sources monitored (LeChevallier and Norton, 1995). Based on this study, Wilson et al. (1996) concluded that all surface waters are or will become contaminated with Crypfosporidium. Thus any potable water system using surface water or groundwater under the direct influence of surface water (GWUDI), as its raw source, is at risk to Cfyptosporidium contamination. The US Environmental Protection Agency (USEPA), by the Safe Drinking Water Act (SDWA), requires all states in the U.S. to identify those groundwaters directly influenced by surface waters, because of their risk to waterborne pathogens. In 1996 the SDWA was amended, forming the Interim Enhanced Surface Water Treatment Rule (IESWTR), which included C. Parvum as an indicator of GWUDI. The Microscopic Particulate Analysis (MPA) was developed by the USEPA to assign a Risk Index (RI) to existing groundwater sources, rating the possibility of surface water influence. This procedure is a collaborative effort to standardize an acceptable measure of microscopic particulates in groundwaters. It is outlined in the USEPA Consensus Method for Determining Groundwaters Under the Direct Influence of Surface Water using Microscopic Particulate Analysis (Vasconcelos and Harris, 1992). Several studies have cautioned against sole I Respectively, Graduate Student, Associate Professor, University of Central Florida, Department of Civil and Environmental Engineering, P.O. Box , Orlando, Florida , mfl7outdoors@hohnail.com 193

2 dependence of the MPA for GWUDl determination (Chin et al., 2000) and suggest the use of a group of indicators. This paper analyzes several hydrogeologic and karst factors as they influence the MPA results cf drinking water wells sampled across the State of Florida, to provide a more confident GWUDl determination. METHODS AND MATERIALS The Florida Department of Environmental Protection (FDEP) has conducted MPA sampling on over 1000 public water system (PWS) wells throughout the state. Eased on the ability to accurately locate these wells in a Geographic Information System (GIS) environment, 719 of these wells were selected for this study. MPA samples were collected from these wells and analyzed at Florida Department of Health (FDOH) laboratories around the state. Sampling and laboratory work strictly adhered to the procedures outlined in the USEPA Consensus Methyd (Vasconcelos and Harris, 1992). Each PWS was then assigned a RI and categorized as having high (20 ), moderate (IO to 19), or low (0 to 9) risk to surface water influence. A detailed discussion of the sampling and laboratory procedures is well documented in Nnadi and Sharek (1999). The distribution of wells with a RI of 1 or greater is shown in Figure 1 with Florida s five water management districts. Figure 1: Positive RI Well Distribution over Florida s WMDs NW FW MD 90- & N,A- S a o 0 High Index Wells Moderate Index Wells Low Index Wells Water Management Districts Miles P Karst Data The occurrence of karst features in the State of Florida is related to the type of geologic cover, thus both were analyzed in this study. A statewide map of surficial geology was obtained from FDEP s online data set. Created by the Florida Geologic Survey (FGS), this map displays the regions in Florida where each type of geology is dominant. A map showing the possibility of sinkhole development was compiled through!he efforts of Several government agencies. This sinkhole risk map rates regions in Florida where sinkholes are more likely to develop. The sampled wells were analyzed spatially with these maps and correlations were drawn. 194

3 Hydrogeologic Data Four of Florida s five water management districts (WMDs) had conducted recharge-mapping projects by the time of this study. Aquifer recharge information for the state was collected from each water management district (WMD) separately, since individual standards were used to represent their data. To date, the Southwest Florida WMD has not compiled a recharge map for areas under its jurisdiction. Analyses were conducted with these maps to determine the distribution of sampled wells over each region of aquifer recharge. RESULTS AND DISCUSSION Results of the MPA show that 396 of the 719 wells sampled had a positive RI, while the remaining 323 wells were assigned a RI of zero. Of the 396 positive risk wells, 258 were designated as low risk, while 107 and 31 were labeled moderate and high-risk wells, respectively. The St. Johns River WMD contained 44% of these wells, while 22% were present in the Southwest Florida WMD. The South Florida and Suwannee River WMDs contained 16 and 15%, respectively, while only 3% were present in the Northwest Florida WMD. Surficial Geology The positive RI wells were located in each region of surficial geology as shown in Figure 2. Over 32% of the high risk wells were present in clayey sand. Shelly sand and clay contained 26% of these high-risk wells, and only 15% of the wells with a low RI. Medium fine siltand sand contained only 10% of the high-risk wells, while 24% of the zero RI wells were located in this type of geology. Over 80% of the high-risk wells were present in areas of shelly sand and clay, limestone, or clayey sand. A High Index Wells Moderate Index Wells o Low Index Wells 0 State Boundary Surficial Geoloov SHELL BFDS = LIMESTON EID OLOM ITE SANDY CLAY AND CLAY LIMESTONE F$& MED. FINE SAND AND SILT PEAT a CLAYEY SAND Figure 2: Positive RI Well Distribution over Florida s Surficial Geology 195

4 Possibility of Sinkhole Development The sinkhole possibility map (Sinclair and Stewart, 1985) is provided as Figure 3 with the distribution of positive RI wells over each of the four sinkhole development regions. In the first region (lowest possibility of sinkhole development) more than half of the wells present have a RI of zero. In regions 2 and 3, respectively, 58 and 64% of the wells have a positive RI. There are also more oositive risk wells than zero risk wells in region 4 (highest possibility of sinkhole development). Figure 3: Positive RI Well Distribution and Possible Sinkhole Development Aquifer Recharge Since Florida s WMDs employed differing standards when creating their recharge maps, the results are reported separately. In the Northwest Florida WMD. wells with a higher RI were generally present in regions with higher recharge to the Floridan Aquifer system. Groundwater regions with moderate to high recharge contained 60% of the moderate-risk wells, but only 44 and 22% of the low and zero RI wells, respectively. There were no high-risk wells present in the entire NWFWMD. For!he Suwannee River WMD. 62% of the wells present in moderate to high recharge zones had a positive RI. Also, forty percent of the high-risk wells were located in areas that receive the greatest recharge. The St. Johns River WMD divided their land into zones of recharge intensity as shown in Figure 4. The discharge zone is defined as areas where no recharge to the Floridan Aquifer exists, because of groundwater levels. In this district, 70% of the wells located in greater than 12 in& of recharge had a positive risk to surface water influence. Regions with 0 to 4 in/yr of recharge, however, contained nearly the same number of positive and zero RI wells. Four separate aquifer regions are present in the South Florida WMD, three of which are surficial systems. Overall. this district provided no positive trend of wells with risk indices present in any of the recharge ranges. 196

5 Miles S A High Index Wells Moderate Index Wells o Low Index Wells 0 District Boundary Recharge Rate (in/yr) 0 Discharge 0-4 = > I2 U Figure 4: SJRWMD Recharge Zones with Positive RI Wells CONCLUSIONS Understanding some of the factors that influence the passage of Cryptosporidium parvum from surface waters to aquifer systems would provide more confident GWUDI determination. The following conclusions are drawn based on the results of this study. It was noted that the maps used for this study were general in nature and could not be used to make site-specific assessments. Surticial geologic formations of shelly sand and clay, limestone, and clayey sand tend to dominate the presence of high-risk wells. Drinking water wells classified with a positive risk to surface water influence were commonly located in regions at greater risk to sinkhole development. Also, areas of greater aquifer recharge generally contained the majority of the moderate and high-risk wells. 197

6 ACKNOWLEDGEMENTS The authors wish to thank the Florida Department of Environmental Protection, specifically Marian Fugitt for her interest and support. The authors also wish to express their appreciation to Saad AlAyyash for his contribution in the area of GIs. REFERENCES Chin, D.A. and X. Qi, Ground Water Under Direct Influence of Surface Water. Journal of Environmental Engineering, 126(6), Haas, C.N., C.S. Crockett, J.B. Rose, C.P. Gerba, and A.M. Fazil Assessing the Risk Posed by Oocysts in Drinking Water. Journal of American Water Works Association, 88(9), LeChevallier, M.W. and W.D. Norton, Giardia and Cryptosporidium in Raw and Finished Water. Journal of American Water Works Association, 87(9), Medema, G.J., M. Bahar, and F.M. Schets, Survival of Cryptosporidium Parvum, Escherichia Coli, Faecal Enterococci and Clostridium Perfringens in River Water: Influence of Temperature and Autochthonous Microorganisms. Water Science and Technology, 35(11-12), Nnadi. F.N. and R.C. Sharek, Factors Influencing Groundwater Sources Under the Direct Influence of Surface Waters. Journal of Environmental Science and Health, A34(1), Sinclair, W.C. and J.W. Stewart, Sinkhole Type, Development, and Distribution in Florida. Florida Geological Map Series number 1 IO, Florida Geologic Survey. Vasconcelos, J. and S. Harris, Consensus Method for Determining Groundwaters Under the Direct Influence of Surface Water using Microscopic Particulate Analysis (MPA). USEPA Report ; US Environmental Protection Agency: Port Orchard, Washington. Wilson, M.P., W.D. Gollnitz, S.N. Boutros, and W.T. Boria, Determining Groundwater Under the Direct Influence of Surface Water. Tech. Rep. Project #605, American Water Works Association Research Foundation: Denver. Colorado. 198