Spatial Adequacy of Nasqan Water Quality Data in Ohio River Basin

Abstract

Water quality data from 30 stations of the National Stream Quality Accounting Network (NASQAN) in the Ohio River basin were examined for spatial adequacy. A direct statistical test of differences between two sample means (for small sample size and unknown population variances) found that for over onehalf of the 53 water quality parameters, differences in water quality could not be distinguished for more than one‐third of the station pairs. An indirect statistical approach, correlating mean water quality to 136 possible explanatory basin characteristics, resulted in reasonable cause‐effect relationships for only 23 parameters. Based on a method developed to determine a spatially adequate sample size, it is recommended that the current bimonthly or quarterly sampling frequency be increased to monthly for all parameters and that the current annual reporting period be increased to four years. Other suggestions to make the system more cost‐effective are included.