Rapid population growth and development of rural properties can increase groundwater nitrate concentrations, and raise concerns about aquifer water quality (USEPA, 1994). This two-part article illustrates the use of GIS as a tool to evaluate groundwater quality changes over time.

Introduction and Background

Sample Points

     The Helena, Montana area, like so many other parts of the country, is being rapidly subdivided and developed, with individual septic systems as the favored choice for private on-site wastewater treatment. Four investigations (U.S. Geological Survey and Lewis & Clark County Water Quality Protection District) were undertaken between 1990 and 2000 to determine if Helena Valley aquifer nitrate concentrations are changing over time 2000 (Briar and Madison, 1992; Drake, 1991; 1995; 1998; Moore, 2000). The studies were designed, in part, to determine if groundwater nitrate concentrations exhibit trends which correlate with population growth and aquifer utilization. Sampling sites are depicted in Figure 1.

     Nitrate was chosen as the sampled chemical parameter because: 1) it is an Environmental Protection Agency (EPA) regulated contaminant, and 2) its presence in groundwater indicates that contamination from anthropogenic sources, such as septic systems, lagoons, feedlots, and fertilizers may have occurred, and that accompanying pathogenic bacteria and viruses may be present (USEPA, 2001(a)). The maximum nitrate concentration set by the EPA for domestic water consumption is 10 mg/l (milligrams per liter) of elemental nitrogen or 45 mg/l expressed as nitrate (USEPA, 2001(b)). The natural background concentrations found in the Helena Valley aquifer range from 0.1 to 0.5 mg/l (Wilke and Coffin, 1973; Moreland and Leonard, 1980).

The Tools

     The recent study used GIS (ESRI's ArcView Spatial Analyst) which supplanted the multiple and cumbersome software packages used for the 1990, 1994, and 1997 analyses and eliminated much of the difficulty in managing, sorting, and treating the data from various sources. Once the data was mapped into GIS, the analysis tools were quick and easy-to-use, generating contours and surfaces in a few seconds (Figures 2, 3, 4 and 5). Statistical information for any of the data sets was calculated on-the-fly within the GIS. The numerical data could be charted or graphed automatically with the push of a button and the quick entry of a few parameters. GIS greatly enhanced the understanding of the data.

1990	1994
1997	2000

Table 1. Areal distribution in square miles for each NO₃^--N concentration value, based on calculated isopleths.

Table 2. Percent change in areal distribution for each NO₃^--N concentration value, based on change in isopleth coverage for each five-year period.

Conclusions

     A major conclusion of the 1990, 1994, and 1997 studies was that measured nitrate concentrations were elevated in regions with high septic system densities (Drake, 1991; 1995; 1998). Between 1990 and 2000, there was a 68 percent overall increase in septic systems permitted in the Helena area. Figure 6 depicts a GIS-generated map showing the total number of permitted septic systems in each section, together with the spatial distribution of nitrate concentrations for the combined 1997 and 2000 sampling events.

     In general, within the study area, elevated nitrate concentrations correlate with areas having the greatest septic system density. In addition, where sufficient data exist, the spatial distribution of nitrate concentrations also correlates with areas that are downgradient (toward Lake Helena) from populated areas. Additionally, areas with minimum nitrate concentrations correlate with areas having few to no sampling points. As a result, the nitrate levels in those areas are not well understood.

     Where consistent data sets were used (1990-1994), the GIS maps confirmed the results of previous studies, that nitrate levels were increasing over time (Drake, 1995). Unfortunately, temporal changes from 1994 to 2000 could not be determined because sampling data were not collected from the same wells for each sampling event.

Other Observations

     Design of future studies can benefit from using GIS to identify an appropriate set of sampling locations for optimum areal coverage. This exercise also highlighted the importance of selecting and maintaining the same set of sampling points for multi-year trend analyses. Finally, GIS layering and mapping provides a tool for visual interpretation that cannot be accomplished with traditional statistical methods.

Rj Zimmer (rjzimmer@co.lewis-clark.mt.us) is the GIS Program Manager for Lewis & Clark County and the City of Helena, Montana, and a Contributing Editor for Professional Surveyor magazine.

Vivian Drake (rvdrake@onewest.net)supervised the Lewis and Clark County Water Quality Protection District for eight years, is co-owner of Drake Engineering Incorporated, a technology development and environmental consulting firm, and is currently pursuing her Ph.D. with James Bauder, LRES Department, Montana State University.

Montana Water Center
http://water.montana.edu/default.htm

Briar, D.W. and J.P. Madison. 1992. Hydrogeology of the Helena valley-fill aquifer system, west-central Montana. U.S. Geological Survey Water-Resources Investigations Report 92-4023. 92 p.

Drake, V.M. 1991. Hydrogeological characterization of the Helena valley aquifer system based upon statistical evaluation of selected groundwater chemical parameters. Montana College of Mineral Science and Technology, Butte. 153 p.

Drake, V.M. 1995. Helena valley aquifer groundwater nitrate concentration trends. Lewis and Clark County Water Quality Protection District. 29 p.

Drake, V.M. 1998. Temporal change in groundwater nitrate concentrations north of East Helena, Montana. Lewis and Clark County Water Quality Protection District. 3 p.

Moore, K.L. 2000. Spreadsheet sample results from 2000 nitrate sampling event.

Moreland, J.A., and R.B. Leonard. 1980. Evaluation of shallow aquifers in the Helena valley, Lewis and Clark County, Montana. U.S. Geological Survey, Water-Resources Investigations Open-File Report 80-1102. 24 p.

USEPA. 1994. Nitrogen control. Technomic Publishing Company, Inc., Lancaster, PA. 1-22.

USEPA. 2001 (a). Office of Water. Current drinking water standards. Technical drinking water and health contaminant specific fact sheets. http://www.epa.gov/safewater/dwh/t-ioc/nitrates.html

USEPA. 2001 (b). Office of Water. Current drinking water standards. National primary drinking water regulations. http://www.epa.gov/safewater/mcl.html

Wilke, K.R., and D.L. Coffin. 1973. Appraisal of the quality of ground water in the Helena valley, Montana. U.S. Geological Survey, Water-Resources Investigations 32-73. 31 p.

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