The 2022 Year 4 assessment followed the Year 3 2020 mainstem assessment and continued to utilize the analyses and outputs of the Northeastern Illinois Integrated Prioritization System (NE IL IPS; MBI 2023a). Biological effect thresholds for five biological condition categories (i.e., excellent, good, fair, poor, and very poor) were developed for 87 chemical water quality, sediment chemistry, and habitat attributes that are more regionally relevant than what has been used previously. For nutrients, this includes not only more refined thresholds for nutrient parameters, but a combined assessment that assembles indicators and parameters responsive to the direct and indirect effects of nutrients. The IPS yields a Restorability factor for impaired sites, reaches, and watersheds and a Threat/Susceptibility factor for attaining sites. In combination with improved stressor thresholds across five condition categories, the IPS has provided more certainty in the delineation and severity of causes and sources of impairment and threats. The primary indicators of the status of the Illinois General Use for aquatic life are the Illinois fish and macroinvertebrate Indices of Biotic Integrity and generally following the guidance in the 2022 Integrated Report (IEPA 2022) with certain exceptions. The status of aquatic life is reported here in an attainment table (Table 1) and expressed as full, partial, or non-support and based on the most limiting of either the fish or macroinvertebrate results. Non-support is further subdivided into non-support fair and non-support poor; the partial support category was added to clarify instances where only one of the two assemblages attained the General Use support thresholds for fish or macroinvertebrates. The 2022 results showed two sites in full attainment of the General Use for aquatic life, up from one site in 2020, but down from five (5) sites in 2018 (MBI 2020b). The fish assemblage was the limiting factor as the macroinvertebrate IBI met its criteria at all sites in the effluent affected portion of the upper mainstem. The 2022 fIBI scores declined at the formerly attaining sites by anywhere from 0.5 to 3.0 fIBI units with one site missing by only 0.5 fIBI units.
The introduction of large volumes of treated municipal wastewater to the mainstem downstream from the upper modified reach continues to benefit the Upper Des Plaines River and aides biological condition by offsetting detrimental impacts of pollution that is exported downstream from the modified upper reach of the mainstem. The biological results downstream from the entry of treated wastewater at site 13-3 were better both in terms of AQLU status and the fIBI and mIBI scores than at the four upstream most sites that were in non-fair non-attainment in 2022 that have been non-poor in prior years. Even with the lower fIBI scores in 2020, the longitudinal pattern was the same – consistent incremental improvement downstream from the Wetland Research riffle and seemingly independent of the entry of large volumes of treated wastewater. This and the accumulation of more poor and very poor exceedances of IPS and other thresholds in the upper modified reach indicates that nonpoint source pollutants are being exacerbated by the poor habitat and hydrological modifications in the upper watershed as the major limiting sources to aquatic life. Observing this phenomenon over four survey years spanning 2016-22 and examining the differences in the annual flow regime over that same period, the results suggest that periodic elevated flow events that approach or exceed flood stage export most of the upstream pollution out of the study area at least temporarily. However, spates of less elevated high flows act to move and deposit this pollution in the lower reaches of the study area. Besides the revelations of the analysis of the permanent DRWW Datasondes, the observations of nonpoint source derived muck substrates by the fish crew at site 16-2 during the first sampling pass following one of these events, but finding different conditions during the second pass later in the summer-fall index period was reflected by a 10 spread in the fIBI score which was the difference between non and full attainment of the General Use for aquatic life in 2022.