Exceedance Frequency Analysis of Contaminants in Streams Under Dry-Weather Conditions in Denton, Texas

Abstract

Percentages of dry-weather stream samples exceeding water quality criteria for ten parameters were compiled for mixed land use watersheds in north-central Texas. Most problematic were total suspended solids (TSS), total dissolved solids (TDS), ammonia, nitrate, phosphorus and copper. Nutrients had much higher exceedance frequency at a sampling station impacted by wastewater discharge. Whereas, TSS and TDS exceedance frequency was highest in predominantly agricultural and rangeland watersheds, and urbanized watersheds respectively. Total dissolved solids was most often exceeded in urbanized watersheds. For several parameters, especially TDS, TSS, ammonia and copper, median concentrations were below water quality thresholds in most watersheds, but exceedance frequency was high. For example, median TSS was less than its threshold in every watershed, but exceedance frequency was higher than 10 % in four of five watersheds – and nearly 43 % in one watershed. This pattern reflects the skewed nature of water quality data; often times, many observations cluster around the lowest values, causing the median to be relatively low, but several (high) outliers form the right-hand tail of the distribution. Results of this study indicate a need to examine exceedance frequency in addition to traditional descriptive measures to better understand dry-weather stream quality in watersheds.