The Relation of Ground-Water Quality to Housing Density, Cape Cod, Massachusetts

Persky, James

Correlation of median nitrate concentration in ground water with housing density for 18 sample areas on Cape Cod yields a Pearson correlation coefficient of 0.802, which is significant at the 95-percent confidence level. In five of nine sample areas where housing density is greater than one unit per acre, nitrate concentrations exceed 5 milligrams of nitrogen per liter (the Barnstable County planning goal for nitrate) in 25 percent of wells. On the other hand, nitrate concentrations exceed 5 milligrams of nitrogen per Ziter in 25 percent of wells in only one of nine sample areas where housing density is less than one unit per acre. Median concentrations of sodium and iron, and median levels of pH and specific conductance, are not significantly correlated with housing density. A computer-generated map of nitrate shows Q positive relation between nitrate concentration and housing density on Cape Cod. However, the presence of septage or sewagedisposal sites and fertilizer use are also important factors that affect the nitrate concentration. A map of specific conductance also shows Q positive relation to housing density, but little or no relation between housing density and sodium, ammonia, pH, or iron is apparent on the maps. Chemical analyses of samples collected from 3,468 private- and public supply wells between January 1980 and June 1984 were used to examine the extent to which housing density determines water quality on Cape Cod--an area largely unsewered and underlain by Q sole-source aquifer.