Utah’s Division of Water Quality is proposing numeric nutrient criteria to protect aquatic life and recreation uses in headwater streams. This page contains background information on this important water quality standard revision. Documentation includes a technical support document (TSD) that provides details with respect to the scientific underpinnings of the criteria. A shorter proposal (2 MB) describes how DWQ translated the data and information in the TSD into water quality standards—summarized below. The proposed rule language can be found in the Board Packet (3 MB) to the Utah Water Board meeting on March 13, 2019. Finally, check out this rulemaking timeline (287 KB) of events that identifies places where you can get involved.
Summary of the Proposed Nutrient Criteria
The proposed nutrient criteria place headwater streams into one of three enrichment tiers depending on whether or not ambient nutrient concentrations exceed two nutrient concentration thresholds. Two lower nutrient concentration thresholds of 0.4 mg/L for total nitrogen (TN) and 0.035 mg/L for total phosphorus (TP) differentiate between low and moderately enriched streams. A higher threshold of 0.80 mg/L for TN and 0.080 mg/L for TP differentiates between moderate and highly enriched streams. Moderate enrichment streams, with nutrient concentrations between the upper and lower thresholds, require measures of ecological condition to determine whether or not enrichment is impairing or threatening a stream’s designated biological uses.
DWQ is proposing a combination of nutrients and responses because streams vary in physical characteristic that influence their relative sensitivity to nutrient enrichment. By using both nutrients and responses DWQ is able to further evaluate relatively low levels of enrichment while also minimizing false impairment determinations. Nutrients can degrade aquatic life uses via mechanisms related to increased growth of plants/algae (autotrophs) and/or microbes/fungi (heterotrophs). DWQ selected bioconfirmation criteria (ecological responses) to address both mechanisms. In the case of plant/algae growth, two ecological responses are not-to-be-exceeded at any headwater stream: (1) a daily gross primary production (GPP) rate higher than 6 g O2/m2/day or (2) an aerial percent filamentous algae cover exceeding 1/3 of the stream bed. Linkages among microbes/fungi, nutrients, and aquatic life uses are less well understood, in part because these processes are more difficult to observe or measure. However, it is possible to measure ecosystem respiration (ER), which captures the net metabolic activities of all stream biota. DWQ proposes a not-to-be-exceeded rate for ER of 5 g O2/m2/day.
Nutrients can also degrade recreation uses. To protect these uses DWQ proposes a not-to-be-exceeded benthic algae concentration of 125 mg/chlorophyll-a (chl-a)/m2, or the equivalent 49 g ash free dry mass (AFDM)/m2. These criteria are supported by the responses from a survey of Utah citizens who were asked whether streams with varying amounts of benthic algae cover represented “desirable” or “undesirable” conditions. These recommended criteria fall just below the point where the proportion of undesirable responses start to increase and should therefore be protective of recreation from the perspective of degraded aesthetics or other factors influencing recreational use decisions.