Much confusion exists regarding the meaning of “impairment” as used for Clean Water Act Section 303(d) listing. In terms of water quality condition, “impairment” is not a “one size fits all” concept. The degree of water quality impairment ranges from slight to severe. Severe water quality problems, although rare, do continue to occur in Iowa. Most of the impairments on Iowa’s draft list of impaired waters, however, do not indicate severely or grossly polluted conditions. Often, the difference between assessing a waterbody as “impaired” versus “fully supported” can come down to contaminant levels in only one of 36 monthly samples or the absence of a few key aquatic species in a stream. Iowa’s water quality standards and numeric water quality criteria—which are the basis for identifying impairments—are designed to be protective of the beneficial uses designated for Iowa’s streams, rivers, and lakes. These criteria are set to warn of potential water quality problems well before anything approaching “grossly polluted conditions” occurs. Many waters assessed as “impaired” for aquatic life uses often continue to support a moderately healthy and diverse aquatic community. Also, the designated recreational uses of many Iowa waters—especially rivers and streams—are impaired by high levels of indicator bacteria, but reports of waterborne illness related to these high levels of bacteria have been extremely rare in Iowa over the last 40 years.
The list of impaired waters is only as inclusive as the various water quality (WQ) monitoring networks in Iowa. In general, when the amount of monitoring data increases, the number of waters on the Impaired Waters list also increases. The identification of impairments is also strongly tied to the state’s water quality standards; therefore, because Iowa does not yet have numeric WQ criteria for nutrients or sediment/siltation, identification of such impairments is relatively rare. Additionally, because Iowa’s WQ Standards also lack criteria for turbidity, any lake or wetland impairments attributed to algae (chlorophyll) or non-algal turbidity are based on violations of the state’s narrative standards protecting against “aesthetically objectionable conditions”. Eventual adoption of numeric criteria for nutrients, chlorophyll, and/or turbidity will likely result in a substantial increase the number of waterbodies on Iowa’s future lists of impaired waters.
A state’s Section 303(d) list of impaired waters is, in part, an accumulation of impairments identified in past listing cycles. Once added to a state list, the impairment is likely to remain on the list. In general, impairments are identified faster than impairments are removed through the TMDL process or due to water quality improvement. Thus, number of impaired waters on state lists tends to increase over time. Also, as more state waters are monitored over time, the number of impairments continues to increase. While states can easily add new impairments, U.S. EPA carefully scrutinizes any state proposal to remove (de-list) impairments from a Section 303(d) list and sometimes rejects state recommendations for de-listing. In addition, a large proportion of impairments (bacteria and biological impairments) are related to non-point sources of pollution. Unless a state has authority and the means to reduce levels of non-point source pollution, the NPS-related impairments will likely continue to reside on the state’s list of impaired waters.
According to U.S. EPA regulations, impairments can only be removed from a state list for specific reasons, including (1) more recent data showing that the impairment no longer exists, (2) discovery of an error in the data or rationale for the original listing and (3) preparation and approval of a total maximum daily load (TMDL) that identifies sources of pollutant loadings and reductions in loadings necessary to fully attain applicable water quality standards. A primary mechanism for removing waters from Iowa’s list of impaired waters has been preparation and U.S. EPA approval of TMDLs.