July 16, 2014

Des Moines Waterways Snapshot

Cool streams on hot summer days draw children like a magnet.  Children are out of school and seeking adventure.  They see streams as an opportunity to explore, looking for stream critters or perhaps simply to cool off by wading in shallow water.

They are typically less aware of the potential physical, biological, and chemical hazards from runoff, failing septic systems, and broken sewer pipes.  So what is the condition of our urban streams?  Are they healthy and safe or simply storm sewer waterways to quickly discharge contaminated water to the major rivers?  Approximately 25 volunteers, together with staff from the Iowa DNR, have sought to answer these questions.

Polk County SnapshotThe Polk County Snapshot volunteers go out on a designated day in the spring and fall, rain or shine, to perform field analyses, collect samples, and make field observations of the physical characteristics of the water and look for exposed or damaged pipes.  They have been doing this since 2004, routinely collecting samples from 30 sites throughout the metro area to answer water quality observations.

The most recent snapshot was taken in May. The good news is that there is little evidence of direct fecal contamination from broken sewer, despite the more than a thousand miles of sewer pipe.  There is little difference in fecal bacteria levels between rural and urban streams.  During rainy weather with runoff, bacteria levels get very high in both urban and rural streams.  During hot dry weather (when children most likely are exploring streams) bacteria levels are low (sewage from broken pipes would cause very high levels of bacteria during low stream flow because of less dilution by stream flow).  Occasional high bacteria levels were observed in locations where yard wastes were raked into the stream.  It is probable that pet droppings were included with the leaves. Urban streams differ slightly from rural streams following a small shower. In urban areas, roofs and streets create runoff to storm sewers and elevated bacteria levels in the stream while a light rain in rural area soaks into the ground with little runoff to carry fecal matter to the streams.  During heavy rain, rural streams generally had higher bacteria levels.  Livestock wastes are essentially untreated and often applied on the ground and not incorporated to reduce loss from runoff.

Nitrate and phosphorus concentrations are usually higher in rural streams.  Exceptions occur during dry weather below the outfall from wastewater treatment plants.  The concentrations decrease downstream because of dilution and biological activity including algae growth.  Bacteria levels at these sites were low, indicating good biological treatment. Very high chloride concentrations (up to 10 times normal conditions) occurred at sites in the spring that implicated road salt as the source.

Arguably, the biggest threat to stream quality and public safety is flash flooding following a heavy rain.  Storm sewers are designed to drain water from streets as quickly as possible.  This greatly increases erosion leading to bank instability and a scouring of the stream bed.   Redesigning storm water discharge structures and the creation of greenways should reduce this threat, improve habitat, and reduce erosion.  Continued observations and stream monitoring will help determine the effectiveness of these activities.

We applaud the diligence of the stream guardians in protecting our urban streams, making them safer, healthier, and more attractive.

Posted by: Gordon Brand No Comments
Labels: , , , , , Posted in Source Water, Water Quality

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