Ground water nitrate‐nitrogen trends in relation to urban development, Helena, Montana, 1971–2003

Abstract

AbstractGround water nitrate‐nitrogen (NO3−‐N) concentrations in the area of Helena, Montana, have increased in both magnitude and extent over the past three decades. It is hypothesized that increases are due to land‐use changes associated with urbanization. Population in the Helena area increased by 28.7% between 1990 and 2000, with a commensurate increase in subsurface waste water disposal. Aquifer NO3−‐N trends were examined using standard statistical methods applied to identical data sets compiled from 10 publicly funded investigations carried out between 1971 and 2003. Although these analyses indicated an overall increase in NO3−‐N over time, conventional statistical techniques applied to flawed data sets are not appropriate for analysis, nor do they illustrate combined temporal and spatial trends of anthropogenic aquifer impacts. In order to use publicly available data collected over decades, geographic information system spatial analysis using inverse distance–weighted interpolation was employed. Isopleth maps graphically depicting temporal changes in NO3−‐N concentrations and distribution throughout the study area (35,340 ha) were created. Analysis of these maps revealed increases in NO3−‐N concentration and extent over each separate decade. NO3−‐N increases were most evident in areas overlying bedrock aquifers and locales with high density and unpermitted septic systems. NO3−‐N concentrations did not appear to be increasing extensively in areas overlying the shallow alluvial aquifer or along major stream corridors. Urban development and concurrent loss of native and agricultural properties were the main changes in land use in the Helena, Montana, area over the 32‐year study period.