The Data

• Three periods between 1975 and 1990 were selected for comparison and analysis: 1975-77, 1980-85, and 1986-1990.
• Only analyses of water from wells for which the depth was known were used; ionic balances were computed for each analysis, and only those analyses with cation-anion differences (in millequivalents per liter) of less than 10 percent were included in the analysis.
• A land-use map for 1973 was obtained from The USGS Earth Sciences Information Center in digital form and was converted to GRASS format prior to analysis. A generalized land-use map for 1987 was generated by project personnel from areal photos.

Illustrations:

• landuse, 1977
• landuse, 1985