Nitrate-nitrogen is a major component of human wastewater. Nitrogen passes through septic systems virtually untreated and is introduced to the underlying groundwater. Nitrate is often used as an indicator of drinking water quality.
A maximum contaminant limit (MCL) of 10 parts per million (ppm) of nitrate as nitrogen for drinking water supplies has been established by the US Environmental Protection Agency and adopted by Massachusetts state regulation. The limit was established to protect infants from methemoglobinemia, or “blue-baby” syndrome, a potentially fatal blood disorder that can occur when too much nitrate limits the amount of oxygen in the blood. Although inconclusive, some health studies have also linked high nitrate levels to certain types of cancer.
The Cape Cod Regional Policy Plan established a nitrogen loading concentration of 5 ppm to ensure that nitrate levels in drinking water will not approach the federal standard. This protection standard had been adopted both locally and at the state level. The quality of Cape Cod’s community public drinking water supply is generally very good, but over the past 15 years there has been a trend toward some degradation.
Between 1993 and 2008, the percentage of public supply wells tested that had nitrate levels at or below .5 ppm (considered very clean) fell from 57% to 42%. During the same period the percentage of public drinking water levels with nitrate levels between .5 and 5 ppm (the RPP limit) increased from 43% to 55%. The percentage of wells testing above the 5ppm standard has varied between 0% and 4% during the past 15 years, and no community public supply wells have tested over 10 ppm.
The 2008 nitrate concentrations in the Cape’s community supply wells are shown below. In general, the wells with higher concentrations are the older ones that are located downgradient of high-density residential areas. However, even well-protected water supply wells have somewhat elevated concentrations of nitrate that is derived from septic systems and other non-point sources.
Small-volume non-community drinking water wells, which are generally shallower, pump less water, and are often closer to septic systems, have shown a greater degradation than the larger and deeper community wells. Since 2000, the number of very clean non-community public wells with nitrate levels below .5 ppm has stayed around 35% compared to 42-44% of the deeper community public supply wells. The non-community wells with levels greater than 5 mg/l ranged from 7-15% compared to 3% of the community wells. The number of non-community wells with nitrate levels greater than the MCL of 10 mg/l has ranged from 2-6. All of the wells exceeding the drinking water limit are located on the Outer Cape where wastewater disposal and private water supply often occur on the same lot. In response to poor water quality, Wellfleet invested in a public waters supply system to serve its central downtown district, and Eastham has begun water supply site investigations as a first step to provide a public water system.