Nutrient dynamics in an effluent dominated reservoir

In arid climates, increasing diversion of surface waters for domestic, commercial and industrial use has led to wastewater effluent dominated streams and reservoirs. Downstream use of these waters for irrigation, drinking water supply, wildlife habitat, and recreation, has led to increasing concern regarding water quality. The objective of this research was to quantify hydrochemical loads and nutrient dynamics in an effluent dominated reservoir, Barr Lake, located northeast of Denver, Colorado, USA.; During the three years of study (1997–1999), Barr Lake received annual loads of nitrate-N of 207,000 kg, 164,000 kg and 138,000 kg respectively. Orthophosphate-P loading to Barr Lake during the study was 30,000 kg, 15,000 kg, and 13,000 kg, respectively. Mass budget analysis indicated that nitrogen was removed from the Barr Lake water column in each year of study (139,000 kg, 135,000 kg, and 109,000 kg respectively). Orthophosphate-P retention was indicated by mass budget analysis (18,000 kg, 600 kg, and 3500 kg, respectively), but inclusion of uncertainty in the mass budget calculation indicated that orthophosphate-P was not retained in Barr Lake during 1998 and 1999.; In-lake nutrient dynamics were quantified using depth discrete sampling, sediment analysis, groundwater sampling and laboratory redox manipulation of sediments. Analysis of sediment taken from Barr Lake indicated that only minor amounts of phosphorus are bound to particulates in the sediment, despite the high water column concentration of orthophosphate. Laboratory analysis of sediment phosphorus behavior under controlled redox conditions indicated that the sediment has the capacity to adsorb more phosphorus than in-lake sampling would suggest. The high pH conditions and low CO₂ partial pressures resulting from photosynthetic activity associated with the high nutrient loading to Barr Lake limit sediment removal mechanisms such as orthophosphate adsorption to iron (hydr)oxides and calcite coprecipitation. Competition for adsorption sites by OH− or SO₄2− is likely the cause of low sediment retention of orthophosphate. Laboratory redox manipulation experiments suggest that organic phase phosphorus is the control on orthophosphate and nitrate is controlled by organic phase removal mechanisms followed by denitrification occurring under moderately reducing conditions in Barr Lake.