Nitrogen in Groundwater
Most groundwater is low in nitrogen. However, in cases with a very concentrated nearby source, groundwater can contain high concentrations of nitrogen.
High levels of nitrogen in groundwater may be a problem for human health. Many people drink well water in the Puget Sound region, and one contaminant is nitrogen, generally in the form of nitrate. The drinking water standard is <10 mg/L for nitrate in groundwater to avoid blue-baby syndrome.
When groundwater enters surface water in rivers and streams, these high nitrogen concentrations can also affect freshwater quality and aquatic life, and can contribute to algae blooms in downstream marine waters.
Nitrogen reaches groundwater through septic systems, manure, and fertilizer applications, as well as from other human and natural sources. Nitrogen goes through complicated chemical transformations as it passes through interfaces between groundwater and surface water, including rivers and marine areas. Within a very small area, nitrate concentrations can change rapidly into nitrogen gas, which reduces the concentration in the water. The little information available on attenuation suggests that this process is very patchy in time and space and therefore difficult to measure at the Puget Sound scale.
The USGS has developed a fact sheet, as well as a map of groundwater vulnerability to elevated nitrate levels in Puget Sound watersheds. This map illustrates the probability of detecting nitrate concentrations greater than 3 mg/L in wells that are 50 feet deep. The map was developed by relating existing nitrate data to factors describing aquifer susceptibility (ease with which a contaminant can reach the aquifer) and contaminant availability (availability of sources of nitrate at or near the land surface).
Groundwater nitrogen reaches Puget Sound through two pathways: direct discharges to marine waters along the shoreline, and as baseflow in rivers and streams.
Nitrogen from direct discharges of groundwater along the marine shoreline
The amount of water discharging directly to marine shorelines is small relative to the total inflow from rivers, although it may be locally significant in the late summer as river inflows decline. The few direct measurements of groundwater discharges to marine areas cannot be extrapolated to the entire Puget Sound shoreline. The best estimate is 100 to 1000 cfs (Vaccaro et al., 1998), based on large-scale water budgets for Puget Sound. This estimate is small, and represents only 0.2 – 2% of the 50,000 cfs of total freshwater inflow to Puget Sound from rivers.
Using available nitrogen concentration data from wells monitored by the Washington State Department of Health, Mohamedali et al. (2011a) estimated that that the groundwater DIN load entering Puget Sound through direct discharge is between 160 – 1,600 kg/d, which is 0.5 – 5% of the total freshwater DIN load to Puget Sound.
Nitrogen from baseflow in rivers
Baseflow contributions of nitrogen to rivers are included within Ecology’s river loading estimates. The portion of nitrogen delivered via rivers through baseflow has not been established and depends in part on the definition of baseflow. Pitz and Sinclair (1999) estimated the proportion of water volume delivered as baseflow using USGS gaging stations throughout the Puget Sound basin. They found that on average, baseflow produces 50 to 74% of the surface runoff to Puget Sound. It is unclear how this translates to nitrogen loads, but there is some evidence to suspect that nitrogen concentrations in some rivers e.g. the Deschutes and Nooksack rivers, have more groundwater influence than others.
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