Nutrients National Synthesis

Nutrient Concentrations in Ground Waters of the United States

Spatial Statistics

Summary statistics were computed for ground-water nutrients grouped by land-use studies (LUSs) (table 1), subunit surveys (SUSs) (table 2), and LUSs and SUSs combined (table 3), using data collected during 1993-1995 by 1991 NAWQA study units. The summary statistics include the median, mean, standard deviation, and number of nonmissing observations. Although the mean is provided, the median is highly recommended for describing average conditions because of the skewed nature of water-quality data. The median is resistant to the effects of outliers. Censored values were converted to one-half the detection limit. Paired, spatially co-located networks such as trinlusur2 and trinlusur3 were excluded from calculations of summary statistics, to prevent spatial bias.

Nutrients for which statistical summaries were calculated include:

  P00608--nitrogen, ammonia, dissolved, mg/L as N
  P00613--nitrogen, nitrite, dissolved, mg/L as N
  P00623--nitrogen, ammonia and organic, dissolved, mg/L as N
  P00631--nitrite plus nitrate, dissolved, mg/L as N
  P00666--phosphorus, dissolved, mg/L as P
  P00671--phosphorus, orthophosphate, dissolved, mg/L as P

Table 1. Summary statistics for land-use studies

Table 2. Summary statistics for subunit surveys

Table 3. Summary statistics for land use studies and subunit surveys ("national" statistics)

Network Statistics

Summary statistics (median, mean, standard deviation, and number of nonmissing observations) were calculated for nutrients and depth to water for each of the 1991 NAWQA ground-water studies. Historical, median water level (if available) was substituted for water level at the time of sampling if the latter water level was missing. Results are summarized by nutrient in tables 4-9 below. Water-level results are shown in table 10. Tabulated statistics are shown in the following order: number of nonmissing observations, mean, standard deviation, and median.

Table 4. Network statistics for ammonia nitrogen, mg/L as N (detection limit = 0.010 mg/L)

Table 5. Network statistics for nitrite nitrogen, mg/L as N (detection limit = 0.010 mg/L)

Table 6. Network statistics for ammonia-plus-organic nitrogen, mg/L as N (detection limit = 0.200 mg/L)

Table 7. Network statistics for nitrite-plus-nitrate nitrogen, mg/L as N (detection limit = 0.050 mg/L)

Table 8. Network statistics for phosphorus, mg/L (detection limit = 0.010 mg/L)

Table 9. Network statistics for orthophosphate, mg/L as P (detection limit = 0.010 mg/L)

Table 10. Network statistics for water level, feet below land surface