Sources and Characterization of Particles Affecting Transparency In
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Sources and Characterization of Particles Affecting Transparency in Grand Lake and Shadow Mountain Reservoir Prepared by: James H. McCutchan, Jr. June 10, 2015 Rpt 339 Table of Contents Introduction 1 Background Information on Grand Lake and Its Water Sources 3 Physical and Chemical Characteristics of the Lakes in 2012 5 Hydrology 5 General Characteristics of Grand Lake and Shadow Mountain Reservoir: 2012 13 Background Information on Transparency in Lakes 30 Measurement of Transparency and Other Optical Properties of Water 30 Factors Affecting Transparency 33 The Transparency Record 51 Secchi Transparency 52 Attenuation Coefficients 56 Turbidity 59 Separation of Transparency Components 63 Dissolved Organic Matter 63 Phytoplankton 63 Non-Algal Particles 64 Spectral Interactions 64 Regression Analysis of Variation in Transparency 71 Sources and Sinks of Particles 79 Import of Particles by Tributaries 79 In Situ Production of Particles 85 Sinks for Particles 90 Mass Balance of Particles 96 Stable Isotope Ratios 102 Conclusions about Transparency 108 Primary Controlling Factors 109 Implications for Management Alternatives 116 References 119 Appendix I. 121 i Summary Northern Water, Grand County, and the U.S. Bureau of Reclamation sponsored a multi- year study of particles affecting transparency in Grand Lake and Shadow Mountain Reservoir. This report summarizes and interprets information that was collected as part of the study. Results of this study will be used to support the development of appropriate management and control measures to improve transparency in Grand Lake and Shadow Mountain Reservoir. This study included three main components: routine monitoring, mapping studies, and watershed studies. Routine monitoring included field measurements, sampling, and analyses for locations on Shadow Mountain Reservoir, Grand lake, and their tributaries at points near the lakes. Mapping studies (2012 and 2014) included field measurements across many locations on the lakes. Watershed studies (2013 – 2014) included field measurements, sampling, and analyses for multiple locations within the Colorado River watershed, as necessary to identify locations of particle sources. Variation in hydrologic conditions over the study period has provided an opportunity to study controls on transparency over a wide range of conditions. Grand Lake is strongly affected by its native water sources during snowmelt runoff, but its characteristics typically change after peak runoff due to the introduction of water from Shadow Mountain Reservoir. The changes include higher specific conductance, higher nutrient concentrations, and higher particle concentrations. The effect of pumped water on Grand Lake was less in 2013 and 2014 than in 2012, but Grand Lake still was strongly affected in 2013 and 2014 because the effects of pumping persist even after pumping has stopped. The optical properties of natural waters are determined by the combined optical properties of water molecules, dissolved substances, and suspended particles. As light passes ii through water, photons are lost through absorption, and scattering increases the probability of absorption. Transparency is the property of allowing transmission of light without absorption or scattering. Transparency in Grand Lake and Shadow Mountain Reservoir was measured as Secchi transparency, which is the depth within the water column to which a reflective disk remains visible to an observer at the surface. Attenuation coefficients were determined from profiles of irradiance (photon flux) over a range of wavelengths with an underwater quantum sensor. With these measurements of transparency and measurements of dissolved and suspended substances in individual water samples, different approaches were used to determine the effects of individual components affecting transparency. Results show that transparency in the lakes is determined largely by concentrations of particles in surface water. In general, non-algal organic particles had the greatest effect on transparency, but inorganic particles and algal particles also were important. Chromatic dissolved organic matter affects transparency but accounts for only a small portion of variance. For a given mass of particles, organic particles (algal particles, non-algal organic particles) had a greater effect on transparency than inorganic particles, but the overall effect of inorganic particles on transparency was only slightly lower than the overall effect of non-algal organic particles or algal particles. The dominant sources of non-algal organic particles, as determined by analysis of stable isotope ratios, were the Granby Pump Canal and the native tributaries. Macrophytes, although abundant in Shadow Mountain Reservoir, were not an important source of suspended particles, and the abundance of macrophytes may increase, rather than decrease, transparency in the lakes. Particles transported by the native tributaries and the Granby Pump Canal often dominate the suspended particles in Grand Lake and Shadow Mountain Reservoir. For the native iii tributaries, particle concentrations tend to increase with discharge, especially for the North Fork of the Colorado River. When flows in the native tributaries are low, the Granby Pump Canal can be the major source of particles reaching the lakes, but concentrations of particles in the Pump Canal are much lower than the highest concentrations carried by the North Fork of the Colorado River. Thus, water carried by the Granby Pump Canal sometimes is a source of dilution for more concentrated sources of particles (e.g., the North Fork of the Colorado River at times of high discharge). Four primary factors ultimately control most of the variation in particle concentrations in Grand Lake and Shadow Mountain Reservoir (Table A). For a given set of conditions, it is possible to predict particle concentrations and transparency in the lakes (Table B). For any set of conditions that produces concentrations of suspended particles greater than ~2.5 mg/L, Secchi transparency is not likely to exceed 4 m. However, because of the stochastic nature of the primary factors affecting particle concentrations, there is considerable variation in transparency for any given time of year, and precise predictions of transparency are possible only over short periods of time. Identification of the primary factors affecting particle concentrations and transparency in Grand Lake and Shadow Mountain Reservoir provides information on possible management alternatives that could be used to improve Secchi transparency in the lakes (Table C). Some of the management alternatives listed in Table C may not be feasible, and some would have unwanted consequences on water quality in Shadow Mountain Reservoir or elsewhere in the C- BT system. iv Factor Mechanism Native flows Particle conc. for tributaries increase with discharge; WRT decreases Season (time of year) Temp. affects algal growth, mixed depth; snow cover affects particle yield Lake depth Resuspension of particles; internal loading of nutrients Farr pumping Pumping changes water source for SMR, GL; affects WRT, mixed depth Table A. Primary factors affecting particle concentrations and transparency in Shadow Mountain Reservoir and Grand Lake. WRT = water residence time; SMR = Shadow Mountain Reservoir; GL = Grand Lake. Factors Characteristic particle Characteristic Secchi Native Farr concentration, mg/L transparency, m flows Season pumping SMR GL SMR GL High Warm On 5 3 1.5 2 Low Warm On 2.5 2.5 2.5 2.5 High Cool On 3 2 2 3 Low Cool On 2.5 2.5 2.5 2.5 High Warm Off 5 2 2 3 Low Warm Off 8 1 1 4 High Cool Off 3 2 2 3 Low Cool Off 3 <1 2 5 - 8 Table B. Characteristic particle concentrations and Secchi transparency in Shadow Mountain Reservoir and Grand Lake for different combinations of factors under current operations of the C-BT system. Secchi transparency is estimated here from TSS, according to the relationship given in Figure 56. Shading indicates sets of conditions with Secchi transparency outside the range of ~2 – 4 m that is typical under present operations of the C-BT system. Factor Management alternatives Native flows Diversion of Colo. R. at high flow (e.g., via Redtop); sedimentation basins Lake depth Control sources of suspended particles in SMR; deepen Shadow Mtn. Res. Season (time of year) Alteration of GL mixed depth; nutrient control of phytoplankton growth Farr pumping Major alteration of C-BT; operational control; line Pump Canal Table C. Some management alternatives corresponding to primary factors affecting transparency. v Introduction Suspended particles, including suspended algae, contribute substantially to light attenuation in Grand Lake and Shadow Mountain Reservoir. A study conducted in the summer of 2009 showed that the concentration of non-algal suspended particles was the most important factor affecting transparency in both lakes at that time (McCutchan 2010). However, the relative importance of algal versus non-algal particles varies seasonally and across years, and the sources and composition of non-algal suspended particles were not well resolved. From the results of the 209 study, it was apparent that a clear understanding of the effects of algal and non-algal particles on transparency in Grand Lake and Shadow Mountain Reservoir under present and future operations would require the study of particles in both lakes and their water sources, including North Inlet, East Inlet, the North Fork of the Colorado River, and the Granby Pump Canal. Storm runoff not associated with tributaries