Rainfall increases concentrations of Giardia and Cryptosporidium through its influence on turbidity, flow volume, and possibly other unidentified factors. An investigation of the variability of concentrations of Giardia cysts and Cryptosporidium oocysts in the Delaware River was conducted in 1996 at Trenton, N.J. A goal of the study was to examine the relationship between concentrations of Giardia and Cryptosporidium and a variety of more easily measured microbial, water quality, and meteorological parameters. A positive correlation (Spearman rank; p < 0.05) was demonstrated between concentrations of Giardia cysts or Cryptosporidium oocysts and 15 other parameters. Increased concentrations of Giardia and Cryptosporidium as well as a variety of other microorganisms were associated with rainfall. The effect of rainfall on parasite concentrations is due in part to increased particulate matter in the water column following surface runoff and resuspension of river bottom and storm drain sediment.

Following passage of the New Jersey Private Well Testing Act, 50,800 domestic wells were tested between 2002 and 2007 for the presence of total coliform (TC) bacteria. Wells containing TC bacteria were further tested for either fecal coliform or Escherichia coli (FC/E. coli) bacteria. Analysis of the data, generated by 39 laboratories, revealed that the rate of coliform detections in groundwater (GW) was influenced by the laboratory and the method used, and also by geology. Based on one sample per well, TC and FC/E. coli were detected in wells located in bedrock 3 and 3.7 times more frequently, respectively, than in wells located in the unconsolidated strata of the Coastal Plain. In bedrock, detection rates were higher in sedimentary rock than in igneous or metamorphic rock. Ice-age glaciers also influenced detection rates, most likely by removing material in some areas and depositing thick layers of unconsolidated material in other areas. In bedrock, coliform bacteria were detected more often in wells with a pH of 3 to 6 than in wells with a pH of 7 to 10 whereas the reverse was true in the Coastal Plain. TC and FC/E. coli bacteria were detected in 33 and 9.5%, respectively, of sedimentary rock wells with pH 3 to 6. Conversely, for Coastal Plain wells with pH 3 to 6, detection rates were 4.4% for TC and 0.6% for FC/E. coli.

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