The Cornell Teaching and Research Dairy Farm was used to study the historical influence of dairy farming on water quality and soil chemical properties. The farm has milked approximately 360 cows for the past 20 yr and is situated on 526 ha of cropland (390 ha utilized for dairy production) near Harford, New York. Mass nutrient balances (N, P, K) were constructed with historical data from 1979 and 1994 for the 390 ha used for dairy production. The amount of imported N increased more than 40% from 1979 to 1994, although there were year-to-year variations, depending on crop yields. Although nutrient balance (imported minus exported nutrients) as a percentage of imported nutrients on the farm remained relatively unchanged during this period, balance of N increased from 43.1 metric tonnes in 1979 to 66.0 metric tonnes in 1994. However, P and K remained about the same because of the reduced use of fertilizers in the 1990s. During the 15-yr period, total milk production increased more than 40% (2502 to 3604 metric tonnes from 1979 to 1994). Analysis of well water suggested that increasing amount of N balance on the farm resulted in increased well NO3-N concentration. The mean of five wells located in the corn fields increased from 3.3 to 7.0 mg/kg in NO3-N concentration, 70% of the EPA upper limit. Soil P increased from 6.0 to 24.0 (kg/ha) during the same period. Soil K did not change. Mass nutrient balances are important in determining the amount of nutrients remaining on farm. This study suggests N, P, and K balance can be used as an indicator of potential for increased NO3-N concentrations in wells and soil P and K levels, respectively.