Douglas Colorado Nitrate Study: Lessons Learned

Warren Brown, PE and Hope Dalton, REM

Tri‐County Health Department (TCHD) permits and inspects individual sewage disposal systems (ISDS). In addition, TCHD reviews new subdivisions to assess whether ISDS provide adequate sewage disposal and do not adversely impact groundwater.

A 2005 study, conducted by the Colorado School of Mines (CSM), evaluated the potential for contaminants from ISDS in a proposed residential development to impact groundwater. The CSM study utilized a modeling approach to assess whether nitrates originating from ISDS may be transported through the vadose zone (the zone between the surface and the aquifer) to the Denver Formation bedrock. Due to uncertainties in the model parameters, a definitive answer could not be given by the model. To address these uncertainties, a groundwater monitoring program was recommended.

TCHD initiated a descriptive study of the potential impacts of aging ISDS on residential wells. The purpose was to assess the nitrate level in drinking water and to determine if certain factors related to ISDS pre‐dispose wells to nitrate impacts.

Douglas County properties built prior to 1973 and served by both a well and an ISDS were included in the study. Wells (n=295) were sampled for nitrate, hardness and conductivity. In addition, several factors that may have influenced nitrate transport from ISDS to wells were studied. These factors included distance from leachfield to well, elevation of leachfield in relation to the well, age of ISDS, pumping and maintenance, soil type of leachfield, and depth of well.

These factors were analyzed using geographic information system (GIS) technology and data from the state engineer’s office, Natural Resource Conservation Service and Soil Data Mart, and the US Geological Survey. The results indicated many of the wells have elevated levels of nitrate and some have exceeded the 10 mg/L health standard. However, the statistical analysis showed only one factor to be significant.