Surface-water quality, Twin Ponies watershed, Pottawattamie and Mills Counties, Iowa
Existing surface-water quality in the Twin Ponies watershed is significantly affected by runoff from agricultural lands that comprise most of the area. Runoff effects include the addition of phosphate, organic nitrogen, fecal bacteria, trace metals, pesticides and notably suspended sediment to streaMflow, because runoff commonly is a transport mechanism for these constituents. Low-flow water quality generally is characterized by dissolvedoxygen concentrations near saturation, dissolved-solids concentrations of about 450 milligrams per liter, suspendedsediment concentrations less than 200 milligrams per liter, and a pH of 8.2. However~ samples collected during runoff contained biological and chemical concentrations 10 to 30 times greater than concentrations obtained from samples collected during periods of low flow. Suspended-sediment concentrations obtained during runoff ranged froM 1,000 to 7,000 milligrams per liter. Iowa Class "B" standards for dissolved oxygen, ammonia, fecal coliforms, chromium, copper, lead, and mercury in streams were exceeded in numerous samples collected during runoff. It is probable that the variations between constituent concentrations in samples collected during runoff and those collected during low flow will be similar after grade-stabilization structures have been constructed on streams and after land-treatment measures have been implemented in the watershed as proposed by the U.S. Soil Conservation Service. Grade-stabilization structures should reduce gully and channel erosion in the watershed by dissipating the erosive energy of streamflow during significant runoff. Land-treatment measures to be implemented in conjunction with the project would help reduce sediment yield to stream channels. With the impoundments~ a decrease in velocity of the in-flowing water should produce a decrease of both the suspended~sediment concentrations and the chemical and biological constituents associated with the suspended sediMent in the impounded water.