Hypolimnetic Oxygenation System for Methyl Mercury Reduction

Sierra Negrete {CSU, Chico} Dave D. Drury, P.E. {SCVWD} Carrie Monohan, Ph.D. {CSU, Chico; Sierra Fund} Stephen A. McCord, Ph.D., P.E. {McCord Environmental} The Site Santa Clara Valley Water District Elemental Mercury Hg(0) Calero reservoir, located in San Jose, California, on Calero Creek, is listed as an impaired water body. The extensive mining done along the Coastal Ranges of California has exposed mercury to the surrounding watersheds, including the Guadalupe River Watershed where Calero Reservoir is located. The reservoir is 2.2 miles long with a capacity to hold 9,934 acre-­‐feet of water. Inorganic Mercury (HgII) Methyl Mercury (CH3Hg) The Construc>on of the Calero Dam in September 1935 (SCVWD,2015) Purpose of Study Par?cle Bound Mercury (HgP) To compare the concentra?ons of dissolved oxygen with the concentra?ons of methyl mercury in the hypolimne?c zone of the Calero reservoir in an effort to gauge the impact of the Hypolimne?c Oxygena?on System used to treat mercury-­‐impacted waters. Methods Profiling Water column profile done using the Hydro-­‐Lab DS5 sonde at intervals of a quarter of a meter to one-­‐meter depth specific to each stra?fied layer of the reservoir. •  Epilimnion: measurements were taken at every meter. •  Thermocline: measurements were taken at ¼-­‐meter intervals Reference map of the Calero Reservoir in San Jose, CA •  Hypolimnion: at one-­‐meter intervals The HOS system is designed to diffuse oxygen into reservoir’s hypolimnion in an Water quality parameters measured during profiling with the sonde were effort to increase the dissolved oxygen level and inhibit the forma?on of methyl recorded digitally and on site: •  Temperature* mercury. For this study, the HOS system operated from May to October of 2014. •  pH * Parameters of •  Oxida?on-­‐Reduc?on Poten?al interest for this study •  Specific Conduc?vity •  Dissolved Oxygen* •  Chlorophyll Sampling Water samples were collected 1-­‐3 ?mes a month for total and methyl mercury throughout the year 2014 (Drury, 2013). •  Epilimnion •  Mid-­‐Epilimnion** •  Metalimnion ** Collected May-­‐Nov •  Mid-­‐Hypolimnion** •  Hypolimnion The Calero Oxygen Diffusion Line Length and Eleva?on (Drury, 2013) The Diffuser Line Diagram (Mobley Engineering, 2014). Water sample analysis •  Total methyl mercury concentra?on-­‐ EPA Method 1630 (prac?cal quan?fica?on limit of 0.050ng/L) The diffusion line is anchored with concrete blocks one foot from the boUom of the reservoir. During the ?me of opera?on, the system diffuses oxygen at a rate of 12.5 scfm through a line extending the length of 1000 b (Drury, 2013). Temperature •  Monthly temperature profiles show that the surface waters in the Calero reservoir ranged from 9°C to 25°C through the year of 2014. •  The metalimnion zone of the water column ranged from 5 to 10 meters. Within the metalimnion a thermocline developed with temperature fluctua?ons increasing from the months of February to September. •  The hypolimnion maintained a temperature range of 9°C to 19°C through the year. Methyl Mercury Produc>on Data Calero Reservoir May 2013-­‐’14 • Oxida>on-­‐reduc>on reac>ons-­‐Elemental mercury Hg(0) is oxidized into inorganic mercury (HgII) where it can aUach to a methyl group or evaporate into the atmosphere and can be reduced back to elemental mercury. Calero Reservoir June 2013’-­‐14 • Methyla>on-­‐demethyla>on processes-­‐Through the methyla?on process, inorganic mercury (HgII) transforms into methyl mercury through bacteria. The reverse of this process is known as demethyla?on. Hypolimne>c Oxygena>on System Calero Reservoir July 2013-­‐’14 Calero Reservoir August 2013-­‐’14 Ver?cal profiles of dissolved oxygen and methyl mercury concentra?ons for the months of May-­‐August during a 2013 and 2014. The Hypolimne?c Oxygena?on System was opera?ng full ?me during the year 2014. Mercury Reduc>on Goal Under the Guadalupe River Watershed Mercury TMDL, Calero Reservoir’s seasonal maximum concentra?on shall be less than 1.5ng of total methyl mercury per liter of water in the hypolimnion (SFCRWQCB, 2008). •  Fine oxygen bubbles emana?ng from the diffuser line rose slowly through the water column, diffusing oxygen into the water while entraining ambient water into its plume. •  The ver?cal profile of dissolved oxygen determines methyl mercury produc?on in the water column and the diffusion interac?on with the sediment bed. •  As dissolved oxygen decreases, methyl mercury begins to form. This response is not immediate but instead the data indicates there is a ?me lag between dissolved oxygen fluctua?on in concentra?ons and the produc?on of methyl mercury. A long-­‐term study of the Hypolimne?c Oxygena?on System at the Calero reservoir is needed to further inves?gate the effec?veness of oxygen diffusion into anoxic regions of lakes and reservoirs and its ability to reduce methyl mercury. Results Historical Concentra?ons of Methyl Mercury in the Calero Reservoir and the Target TMDL of 1.5ng/L (Drury, 2013). Discussion Conclusion Oxygen Diffusion Line Dissolved Oxygen •  Dissolved oxygen concentra?ons were maintained equal to or greater than 1.0 mg/L at or above the eleva?on of the oxygen diffuser during the months of May-­‐October, 2014 when the system was in opera?on. Methyl Mercury •  Methyl mercury concentra?ons in the hypolimnion ranged from undetected concentra?ons to 0.5 ng/L throughout the year of 2014 and remained below the TMDL target concentra?on of 1.5 ng/L. (Ministry for the Environment, 2008 and USEPA, 2014) References A comparison between dissolved oxygen and methyl mercury concentra?ons in the Calero reservoir during 2013, the year before full-­‐
?me opera?on of the Hypolimne?c Oxygena?on System . 1.  California Regional Water Quality Control Board San Francisco Bay Region (SFCRWQCB). Guadalupe River Watershed Mercury Total Maximum Daily Load (TMDL) Project: Basin Plan Amendment. 2008. 2.  Chrystall, Leila. et Al. Ministry for the Environment. Mercury Inventory for New Zealand: 2008. Mercury in the New Zealand Environment:2 Mercury in the Environment. 2009. 3.  Drury, Dave. Methyl Mercury ProducLon and Control in Lake and Reservoirs Contaminated By Historic Mining AcLviLes in the Guadalupe River Watershed. 2013. 4.  Mobley Engineering. Line Diffuser OperaLng and Maintenance Manual with System OperaLon RecommendaLons. 2014. A comparison between dissolved oxygen and methyl mercury concentra?ons in the Calero reservior during 2014, the year of full-­‐?me opera?on of the Hypolimne?c Oxygena?on System. 5.  United States Environmental Protec?on Agency. Mercury Study Report to Congress. Volume III: Fate and Transport of Mercury in the Environment. 1997.