Coliform Bacteria in Bedrock Groundwater: Insights from the NJ Private Well Testing Act Database

It has now been over 10 years since enactment of the NJ Private Well Testing Act (Sept 2002).  As of Sept 2012 the NJDEP has accumulated water quality data from over 80,000 wells, and over 93000 samples (including wells sampled more than once).  Each sample was analyzed for a variety of parameters, including coliform bacteria.  The database has some limitations with respect to data quality, but there is no sampling bias with respect to water quality and very little bias with respect to sampling time or location.  Analysis of the coliform bacteria data revealed a number of factors that affected the bedrock detection rates.  These factors include; the laboratory and analytical method used, the number of times a well is analyzed, season, precipitation, and geology.  Based on a single sample per well, bedrock detection rates (percent positive wells) were 20% for total coliform bacteria and 3% for fecal coliform or E. coli bacteria.  Wells located in sedimentary rock were more vulnerable to coliform contamination than wells located in igneous or metamorphic rock and wells with low-pH water (3-6) were more vulnerable than wells with higher-pH water (7-10) regardless of rock type.  Wells located in areas with thick glacial sediment were less vulnerable than wells located in areas with little or no overburden.  Both temperature and precipitation appeared to affect monthly coliform detection rates, which are higher during warm months.