Summit County Water Quality: Septic Systems and Potential Nitrate Pollution Analysis

This study demonstrates the use of a geographic information system (GIS) for modeling septic system nitrate impacts to water quality in the upper Blue River watershed, Summit County, Colorado. A GIS was designed to integrate Summit County land parcel boundaries, septic system permitting data, geology, soil types, and residential well water quality sample results. This study resulted in multiple ArcInfo coverages for the Summit County Environmental Health Department and the Summit Water Quality Committee to use in planning.

ANALYSIS AND APPLICATIONS

SEPTIC SYSTEM SITE CHARACTERISTICS AND NITRATE POLLUTION POTENTIAL

The thesis entitled "A GIS to investigate septic system site characteristics and nitrate pollution potential" prepared by Stacey L. Stark is the complete source for these methods and results. This thesis is available from the Department of Earth Resources, Colorado State University, Fort Collins, Colorado.

HYDROGEOLOGIC RESPONSE AREAS

Water quality in surface waters is determined by hydrological responses that vary geographically. The subsurface hydrologic environment has the primary influences on groundwater movement and hence pollutant migration to the surface water. Summit County has complex and varied geology and therefore topography and soils. Using the 1:500,000 scale digital geologic map by Green (1992), five geologic groups in the upper Blue River basin were separated out based on their ability to transmit water. The actual transmissivities of these formations is not known, but the ranges of hydraulic conductivity, water supply data, and texture descriptions were used to classify the groups. These groups, ranging from highest to lowest transmissivities, are the Quaternary Age alluvium deposits, Tertiary Intrusive crystalline rocks, Shales from Cretaceous Age, the Sedimentary Bedrock, and Metamorphic - Igneous rocks of Tertiary Age.

Each polygon associated with the 1:24,000 scale soil survey maps was given attributes for permeability, depth to high water table, hydrologic soil group, and depth to bedrock. Ten unique soil groups were then distinguished manually by these characteristics.

Different hydrogeologic units were created from the intersection of the geology and soils coverages. Each hydrogeologic response unit will have a different potential for groundwater pollution due to different hydrologic response, providing an effective unit to describe source areas as well as relative rates of attenuation and travel of nitrate from its source to the surface waters. Nitrate source areas were defined as adjacent land parcels using on-site wastewater treatment within each unique hydrogeologic area.

NITRATE POLLUTION POTENTIAL

Nitrate pollution potential at the source area was rated by determining relative potential groundwater impact at the source using the DRASTIC groundwater vulnerability model, implemented with Arc Macro Language (AML). The EPA and the NWWA's DRASTIC model is an aquifer vulnerability model used for mapping pollution potential on a regional scale (Aller et al., 1987). A higher DRASTIC index will indicate higher vulnerability when interpreted in a specific hydrogeologic setting over a regional scale.

The DRASTIC index was first calculated within each soil and geologic group intersection (the hydrogeologic units). The groups of continuous parcels using septic systems and in the same hydrogeologic unit type were also used as a unit of analysis. The delineation of these units is illustrated below.

These new units of analysis were termed "clusters". The clusters represent not only a unique hydrologic unit, but also a unique location. The cluster units of analysis are defined as adjacent parcels of less than 10 acres each, using septic systems, and in the same hydrogeologic unit. The "cluster" units of analysis were rated using the DRASTIC model using the soil and geologic group characteristics but also local characteristics of depth to water table and slope.

The DRASTIC index values ranged from 37 - 88 for the hydrogeologic units and from 35 - 84 for the clusters based on a scale of 100. The average rating for the hydrogeologic units was 58 and for the clusters, 55. The areas in alluvial fill or in cumulic cryaquoll soil groups stand out as being most vulnerable to nitrate pollution using the DRASTIC model because the depth to water and impact to vadose zone ratings were high for these groups.

CORRELATION OF DRASTIC INDEX WITH RESIDENTIAL WELL SAMPLES

The median nitrate levels were regressed with averages of each factor using each hydrogeologic unit or cluster as an observation. The median nitrate level in each hydrogeologic unit was regressed with its corresponding groundwater vulnerability rating to test this assumption. None of the regressions resulted in an r-squared value that was greater than 0.5.

There are several possible reasons why the expected relations between septic system site characteristics and nitrate levels were not observed. In general, the use of drinking wells to estimate nitrate contamination to groundwater from septic systems will greatly underestimate the groundwater contamination occurring because, presumably, the wells and the leach fields are designed to minimize any contamination to the well. Second, in accordance with the properties of soil and water media previously discussed, the distance and the material through which the contaminated water flows determines the amount of nitrate in the groundwater at any given point. Therefore, the hydrologic distance of the septic system to the well and the depth at which the water sample is taken are critical factors to interpreting the measured nitrate levels. Furthermore, the distance and relative position of the well to the recharge area must be more completely understood to evaluate the well nitrate sampling data. The water sample at the well may not be influenced by the closest nitrate source if a recharge area distant from the well is supplying its water. It is possible that the units and clusters containing only a few nitrate samples may not have provided a good representation of the water quality in that unit or cluster.

A CONCEPTUAL MODEL TO RELATE SOURCE AREAS TO SURFACE WATER QUALITY

A conceptual model was developed in part to explore the data needs and procedures necessary if the Summit Water Quality Committee and the Summit County Environmental Health Department wished to directly relate septic system site data to nitrate levels in the stream. A flowchart was developed to summarize the process involved in the conceptual model. The detailed description of this model can be found in the Thesis, "A GIS to investigate septic system site characteristics and nitrate pollution potential" (Stark, 1997).

Acknowledgments:
Summit Water Quality Committee and Summit County Environmental Health Department

Publication
Stark, SL, Nuckols, JR, Rada, J. 1998. Using GIS to investigate septic system site characteristics and nitrate runoff potential. Journal of Environmental Health 61(8): 15-20 ff.

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