As the Big Sioux River hits Sioux Falls, TSS and Nitrate levels are still below the established standards, but TSS numbers are climbing and even occasionally spike above the established standard. While TSS is worse at I-90 than at the upriver site, E. Coli numbers are much lower; however, despite the improvement, these numbers are still significantly higher than the established standard.

After the Big Sioux exits Sioux Falls and flows through Brandon, numbers are higher. Average TSS readings only barely clear the established standard. E. Coli readings exceeded safety levels in 12 of our 17 samplings, peaking at 7700 in late June. Nitrate readings were low due to the high water flows diluting their concentration. 

The best reading for what the Sioux Falls metro area does to the water quality is found at Falls Park. For the first time, average TSS levels rise to equal the established safety standard. Several TSS readings exceed the standard. Beginning in mid-June, E. Coli levels stay above 126MG/L through the summer and into the fall. Nitrate levels are low. Skunk Creek, a major tributary of the Big Sioux in Sioux Falls, has similar test results with unsafe E. Coli levels, high TSS levels, and low nitrate levels. This indicates that there is still work to do implementing conservation practices in the Skunk Creek watershed. It also indicates that there is runoff from the urban footprint of Sioux Falls, adding to the contamination. 

South of Sioux Falls, the river experiences a significant increase in all three contaminants as it flow through a primarly agricultural landscape. When flood levels dipped, allowing access to test sites, results reveal high E. Coli levels from late May through mid-August. TSS levels are high for June and July, declining in August as flood water receded. Nitrate levels, while still below the safe level, are nearly doubled from earlier site readings. 

Working its way further south down the Iowa-South Dakota border, the river continues to deteriorate. E Coli readings never fall below the safe level, and the TSS level only once falls below the established standard. Nitrate levels climb to nearly half the established safe standard and peaks at 8MG/L in late June. Nitrate test levels started high as fall fertilizer applications ran off due to lack of crops to take up the fertilizers. As the growing season progressed, nitrates from tile drainage was the likely cause as this area is heavily tile drained. 

• The Big Sioux River consistently exceeds established E. Coli standards – E. Coli contamination results from fecal matter—human, pet, wildlife, and livestock. Pet waste from local parks, cattle in the streams, manure runoff, and failing septic systems are likely E. Coli culprits.
• The problems facing the Big Sioux come from both urban and rural sources – It is easy to point fingers across city lines. However, this data shows that water flowing through rural areas becomes more contaminated and water flowing through urban areas becomes more contaminated. In order to attain a cleaner river, both sides will have to take responsibility and—more importantly—action.
• Heavy precipitation is hard on our river – Following rainfall events, contaminant levels spike across the board, revealing a serious runoff problem. Most of our pollution is not coming from a single, point source but rather a community of polluters who make small decisions that have big consequences.
• Late June to early July saw the highest spike in contaminants – A combination of factors could be blamed for this window seeing the highest E. Coli, TSS, and Nitrate levels. Between lawn care and agricultural practices, coupled with lots of rain in those weeks, there was a continual source of contaminants and trash washed into the river. 
• We have a lot of work to do – With contaminants failing to meet established standards, we need to see this data as the canary in a coal mine. Without work, these numbers will continue to rise until the river is more of a contaminant soup than water. Simple actions can help curb these numbers. Keep animals and animal waste out of the water to reduce E. Coli levels. Avoid tile drainage to keep nitrate levels low; demand local industries keep their nitrate discharge to the 10MG/L safe standard. Finally, keep plastics, lawn clippings, fertilizer, salt, sand, and other contaminants out of your storm drain and out of the river.