A quantification of groundwater seepage during drought and

A quantification of groundwater seepage during drought and its importance for water quality modeling in the St. Vrain watershed Hannah Chapin Thomas Gerber

Outline 1. Drought: historical and environmental perspectives 2. Water quality and TMDLs 3. Our case study 4. Conclusions and 5. implications

Water in Colorado • All water from precipitation (no inflowing rivers) • Colorado is dry • Usable water comes from precipitation (snow) that forms a snowpack, melts into surface water, and flows down to where it can be captured and used • All water is highly managed - water rights are precious, and any one stream has hundreds, if not thousands, of individual water inputs and outtakes

Water in Colorado’s History • Variable periods of dry and wet • Colorado was settled during a wet period (1905 -1929) • 1930 -1940: most widespread, longest lasting drought • 1974 -1978: most recent multi-year drought • 1982 -1999: most drought-free period since 1890, second longest wet period in recorded history The recent wet period has seen rapid growth in population and development, major changes in water use, and is our main statistical record and collective memory about water availability. It is NOT, however, representative of the overall history of water in Colorado.

Colorado in Drought with Low Streamflow Colorado: Severe hydrologic drought during the summer of 2002, with unusually low streamflow States where average streamflow was below normal during the previous 7 days http: //water. usgs. gov/waterwatch/W_dryw_map. html

Drought of Summer ‘ 02 • Snowpack as of May was 13% of the average water equivalent • Water supply 70% of normal Next year predicted to be 39% of normal • Third consecutive year of drought worse due to snowpack, runoff, precipitation, temperatures

State Response State response has been guided by the Colorado Drought Response Plan. • created 1981 • provides for a system of monitoring, impact assessment, and response to severe drought on both the sate and local levels. In the recent drought, the plan was activated on April 22, 2002 and Colorado was declared a Drought Disaster area May 30, 2002

Boulder Community Response Boulder responded to the drought by creating mandatory water restrictions, which were effective (see graph, right). There was, however, public uncertainty about the severity of the drought due to a lack of information and/or conflicting media reports. There was also a sentiment that the city could be overreacting. Finally, reducing water use also reduces discharge into urban streams, decreasing flow even further, and further complicating the impact of community response.

What is Water Quality? · A departure from some ‘natural’ background? - A problematic notion · Context is key: what we value - Water quality defined in terms of beneficial use

The Clean Water Act 1972 Two major objectives: 1. Set surface water quality standards 2. Regulate discharge of pollutants * States responsible for implementation

Water Quality Standards • Established to protect designated uses for surface waters • Standards vary depending on use designation • Uses include: Recreation Agriculture Water Supply Aquatic Life

Regulation of Pollutant Discharge · National Pollutant Discharge Elimination System (NPDES) 1. Issues permits to individual facilities discharging pollutants from any point source in the U. S. 2. Defines Pollutants A) Conventional B) Toxic C) Nonconventional (ammonia)

Ammonia Toxicity • Ammonia (NH 3) is toxic to fish • % unionized ammonia and toxicity controlled by p. H and temperature • Total ammonia removed by nitrification

Wastewater Treatment Process: The basics • Ammonia arrives at the plant in wastewater • Secondary treatment: Microbial respiration creates ammonia • Tertiary: Nitrogen trickling filter removes ammonia The difference in both cost and quality of effluent is not trivial!

Water Quality Impairment • CWA 303(d) list: Requires states to biannually review and submit ‘water quality impaired’ segments of surface waters to EPA • Impairment occurs when standard based on designated use is exceeded • Once listed, the state Water Quality Division must develop a TMDL

Wyoming Nebraska South Platte Watershed Colorado St. Vrain Watershed

Water Quality Limited Sections: St. Vrain Watershed, Colorado. Impairment caused by ammonia

The Total Maximum Daily Load Two key meanings of the TMDL • The TMDL is a process used to implement state water quality standards • The TMDL is a quantity, or the assimilative capacity (AC), consisting of: Point-Source Allocation (PSA) Nonpoint-Source Allocation (NPA) Margin of Safety (MOS) TMDL = PSA + NPA = AC – MOS

St. Vrain TMDL Goals Science • Establish the watershed assimilative capacity Policy • Distribute assimilative capacity • Consider all stakeholders - Point Source (permit holders) - Nonpoint source

The St. Vrain TMDL Inputs • Addition/removal of water and ammonia • Influences of p. H and temperature on toxicity • Biological conversion of ammonia

1) Ammonia model: CAM • Spreadsheet model • Used by state of Colorado for permitting dischargers

2) Flow Model KEY: Assume worst case for ammonia occurs when flows are lowest. • Analysis of flow records over the last 10 years • Calculation of residuals from water balance • Assume this unmeasured component is diffuse seepage. • Combine with low flow regime