In 2016, the Colorado Water Quality Control Commission added the 67-mile Timpas Creek to the Clean Water Act Section 303(d) list of impaired waters because sulfate levels impaired its drinking water use. Seven years later, voluntary efforts by local producers to improve irrigation practices helped reduce sulfate loading from irrigated cropland activities. The work was concentrated in the lower 24-mile section of the creek, where irrigated agriculture is practiced.The section now meets the drinking water use water quality standard for sulfate and was removed from the impaired waters list following Colorado's 2023 Regulation 93 rulemaking hearing.High sulfate levels led to the listingTimpas Creek is a perennial stream in the Lower Arkansas watershed and is designated for drinking water, aquatic life, recreation and agriculture.Approximately 95% of streams in the Lower Arkansas River watershed are impaired by at least one regulated pollutant, including selenium and sulfate.Data assessed between 2009 and 2011 showed a 50th-percentile sulfate concentration of 1,390 milligrams per litre, well above the 250 mg/L standard used for the 2016 impairment listing.You Might Also Like:Sulfate pollution, commonly associated with fertilizer use, is present in the lower portion of the watershed, where irrigated agriculture has been practiced. The creek also runs over a shale deposit containing limestone, which can contribute to higher sulfate concentrations. Sulfate from the shale deposit enters the groundwater system and gradually seeps into surface water.Producers adopted more efficient irrigation systemsSeveral irrigation improvements were installed in the Timpas Creek watershed, with the Colorado Nonpoint Source Program providing funding and technical assistance. Most of the BMPs were installed by private stakeholders, with or without contributions from federal programs such as the Natural Resources Conservation Service's Environmental Quality Incentive Program.The restoration work primarily focused on converting flood irrigation to sprinkler and drip irrigation systems.These systems allow producers to combine irrigation and fertilization in a single application through fertigation. They can improve water and fertilizer application efficiency while reducing runoff, soil erosion and deep percolation.You Might Also Like:From 2010 to 2021, another 21 drip-irrigation systems and 14 sprinkler systems were added in the watershed. Drip-irrigated land increased from 598 acres to 882 acres, while 731 acres of flood-irrigated fields were converted to sprinkler irrigation.The Colorado Nonpoint Source Program awarded $119,000 to install roughly 40 acres of drip irrigation systems and $135,000 to install 5,400 linear feet of lined earthen canal. Together, these projects generated $169,333 in local match.Sulfate levels fellMonitoring data collected between January 2016 and October 2019 showed a 60th-percentile sulfate concentration of 890 mg/L, representing a 36% reduction from 2010 to 2019.A water quality standard of 1,145 mg/L was used in the 2021 assessment. The original 2016 listing assessment used a 50th-percentile sulfate concentration of 1,390 mg/L, which was still higher than 1,145 mg/L.The removal of sulfate from the Section 303(d) list was therefore attributed to the significant reduction in sulfate concentrations in Timpas Creek rather than to the use of a different standard.Phosphorus levels also improvedThe BMP implementations also resulted in improvements in total phosphorus concentrations.Data showed a 64% decline in total phosphorus concentration between 2016 and 2020. This could be attributed to changes in irrigation methods that reduced runoff and erosion carrying phosphorus into water bodies.Timpas Creek remains listed for dissolved selenium, total arsenic and total iron. Continued implementation of BMPs, including further irrigation efficiency improvements and additional canal and pond lining, will be important for reducing sulfate and other pollutants and achieving water quality standards.