Nitrogen in Drinking Water Supplies

 

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 ppm of nitrate as nitrogen for drinking water supplies has been established by the USEPA and adopted by MA state regulation. The limit was established to protect infants from methemoglobinemia or “blue-baby” syndrome, a potentially fatal blood disorder which 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 Barnstable County 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 (Figure 7).

Figure 7

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%-42%. During the same time period, the percentage of public drinking water levels with nitrate levels between .5 and 5 ppm (the RPP limit) increased from 43%-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 10ppm.

Figure 8 shows the 2008 nitrate concentrations in the Cape’s community supply wells.  In general, the wells with higher concentrations are the older ones, which are located downgradient of high- density residential areas. However, even well protected water supply wells have somewhat elevated concentrations of nitrate which 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.

Figure 8

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