Upper Los Pinos Water Quality Report

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Ice 5u:dir, cir~n3asing° FpiRi.mnian wbtm water Vr.rPii%rl ---- ---^--^ t'c~nieal .iruihLOn Thermarlinc o rulao~rn ©e~fh ---------- ~, 17ark,. ~ } stagnant, eaaRer Hypalimnitin roarer R •l,r ti.rinf 7hnnal 4 i. '.al sra¢raru3r ,u n xtra nSics[ion nr.. J-tl~:i Figure 12. Schematic of thermal stratification and circulation (overturn) in a lake. eutrophic lakes decreases in dissolved-oxygen content (or becomes anaerobic). Hydrogen sulfide and dissolved iron and manganese concentrations can be released from bottom sediments as a result of anaerobic conditions. Mercur-~T also can be released into the dissolved phase through a process called methvlation. Mercui~T is insoluble in aerobic conditions and remains attached (sorbed) to bottom sediments; ho~yever, in anaerobic conditions, methyhnercuiv (CH3Hg) forms from the biological reduction of carbon dioiide and hydrogen in lakes with low dissolved-oiygen content--usually in the bottom of anaerobic lakes. Meth~Thnercuiv is soluble and is of great concern because it bioaccumulates in fish, such that higher-order fish accumulate the mercury that is ingested by its food sources. Water-quality data and field ~yater properties were collected in Vallecito Reservoirby the USGS and the Volunteer La1ce Monitoring Program. A multi- paranneterwater-quality sonde was used to collect profiles ofivater properties in the lake. Although data were collected for the southern (deepest) part of the lake, the middle of the lake. and the northern (shallowest) part of the lake. the most data eiists for the southern (deepest) part of the lake; therefore, onh~ data for the southern (deepest) part of the lake will be shown in this report. Graphs of the lake profiles indicate that water properties change by season and year (fig. 13). Specific conductance values increased after the Missionan~r Ridge wildfire (August and September 2002 data points. figure 13B). Dissolved oxygen concentrations ~~-ere very low (almost zero) at the bottom of the reservoir durilig August and September 2003. This may have been caused by organic matter that washed into the reservoir duri<ig the year following the wildfire. Logy dissolved- ozygen concentrations were attributed to the cause of a fish kill (mostly- kokanee salmon) in Vallecito Reservoir during August 2003 (M. Japhet, Colorado Division of Wildlife. oral commun., December 2004): however, dissolved-oxygen concentrations also were yen- low at the bottom of the reservoir- during September of 2000 (data not shown). There ma}- have been other factors involved with the fish kill of 2003, such as increased iron and manganese concentrations in the waters of the reservoir; however, samples for dissolved-metal analyses have not been collected ili the reservoir since 2000. Graphs showing variation ofwater-quality parameters over time at the bottom of the reservoir indicate the effects of the Missionai-~- Ridge wildfire (fig. 14A-14C). Specific conductance values and dissolved-ammonia concentrations were highest immediately following the fire (fig. 14A and 14B). Dissolved-oiygen concentrations were lowest duruig 2000 and 2003 (fig. 14C). Because of the low reservoir elevations during the drought of 2002. water temperatures were highest during 2002-2003 following the wildfire. The highest water temperatures correspond to the highest specific conductance values during 2002-2003 (fig. 14D). The highest specific conductance values correspond to the lowest reservoir elevations (fig. 14E). Low dissolved- oiygen concentrations in the reservoir are not necessarily related to the occurrence of the wildfire (fig. 14F). A graph showing variation of specific conductance over time at the surface of the reservoir also shows the effects of the ~yildfire (fig. 15A). Because of the low reservoir elevations during the drought, specific conductance values were highest following the fire (fig. 15B). The visual depths at ~yhich a sir-inch disk (Secchi disk) can be seen decreased following the ~yildfire (fig. 1~C). A logy secchi depth also occurred before the wildfire. It is signuficant to note that specific conductance values increased innnediately follo~ying the wildfire and remained high during subsequent years (fig. 14A). Specific conductance values in Los Piiios River do~~nistreann from the reservoir also remained high following the ~yildfire (fig. 11). Research data have been collected in Vallecito Reservoir regarding mercunr concentrations in fish tissue. These data have been collected by the State of Colorado because of human health concerns through the eating offish caught in the reservoir. The data were not available to the public at the time of the writilig of this report. ~' ~ SouthwestHydro-Logic 25