Nutrient Source Study of Clear Lake

 

Workplan

Study Site 2. Stream flowing into Clear Lake

ARTICLE I. Statement of Contract Purpose:

To provide the Cerro Gordo County Soil and Water Conservation District (CG) and the Iowa Department of Natural Resources (DNR) with a detailed nutrient budget of Clear Lake, Cerro Gordo and Hancock Counties, Iowa for the purpose of planning a restoration program on that lake and its watershed.

 

ARTICLE II. Project Introduction:

Preliminary studies have shown that Clear Lake is being impacted by excessive phosphorus and nitrogen inputs from agricultural and urban sources. These studies have indicated loading problems, but have not produced a nutrient budget for the lake or identifies specific areas of the watershed supplying excess nutrients. Discussions between members of the CG, DNR personnel, and the Iowa State University (ISU) personnel have identified the following information needs:

1. Knowledge of the amounts and sources of phosphorus (P) loading to Clear Lake via surface flow

At CG and DNR's request, we propose to expand the existing rural watershed monitoring scheme (currently 7 sites, including 5 agricultural drainage tiles) into a structured urban and rural monitoring program to fill these needs. The first step in this monitoring program will be to identify potential subwatersheds for study in both urban and rural areas of the larger watershed. Existing maps and other data sources will be used to accomplish this task. Subwatersheds will be selected to represent different land uses, basin sizes, and drainage patterns that are formed within the Clear Lake watershed. CLEAR Coalition representatives have identified approximately 70 storm sewer drainages into Clear Lake, of which we plan to select up to 20 that will represent mixes of residential, commercial, and green-space areas in urban sub-basins. Two rural subwatersheds will be identified and sampled to assess nutrient load characteristics of surface, subsurface, and septic tank contributions to Clear Lake. By monitoring nutrient export from selected subwatersheds, it will be possible to determine specific sources of P loading and to estimate a surface water and nutrient budget for the lake. Further, streams and tile drainages in selected rural areas will be sampled to determine differences in P transport via surface and subsurface flow. Tile drainage from areas with and without public sewer access will be sampled for caffeine and chloride to determine sources of human effluent-derived P. Urban storm sewers will be sampled to determine the relative contribution of urban versus rural P loadings. Following the completion of field sampling, we will prepare a final report outlining a nutrient budget of Clear Lake and the relative nutrient contributions to the lake from specific areas of the watershed.

 

ARTICLE III. Description of Work and Services

1.Tributary monitoring: Sampling stations have been identified on inflow streams (sites 11, 14-17, 19, 20), ephemeral channels (sites 24, 26, 27), marsh outflows (sites 18, 23, 25), and the lake outflow (site 41) and are shown on the accompanying map. samples will be taken biweekly during the months of June-September, 1998 and April-June1999, as well as monthly from October, 1998 to March, 1999. Water samples will be taken from both the lake and tributaries (when they are flowing) on these regular sampling dates. Additionally, these tributary stations are to have samples collected from them within 24 hours of precipitation events that produce more than 1 inch of rain. Precipitation records for the last 40 years indicate that an average of eight (range 3-18) 1 inch rain events occur in Clear Lake annually. We have budgeted to analyze samples collected from these stations for 20 regular sampling occasions and 13 rain events over an annual period. Collected samples will be analyzed for total phosphorus, total nitrogen, and total suspended solids. Total phosphorus and total nitrogen sample analyses will be run in triplicate as part of the Quality Assurance/Quality Control (QA/QC) protocol. GC volunteers will collect samples and flow data from these 15 stations for all samples rain events and for 15 of the 20 regular sampling occasions. ISU will collect samples for the other 5 regular sampling occasions in order to determine discharge rates. ISU will provide training, sampling bottles, and equipment for CG volunteers.

2. Rainwater, field tile, and storm sewer monitoring: Sampling stations have been identified for field tile (sites 12, 13, 21, 22) and "north shore" storm sewers (sites 1-10) on the accompanying map. Ten possible "south shore" storm sewers (sites 28-37) have been identified and are shown on the accompanying map. Rainwater sampling locations have not yet been identified but six locations will be spatially distributed within the watershed. These stations are to have samples collected from them within 24 hours of precipitation events that produce more than 1 inch of rain. We have budgeted to analyze samples collected from these 30 stations for 13 rain events and 5 samples from spring snowmelt runoff. Collected samples will be analyzed for total phosphorus, total nitrogen, and total suspended solids. Total phosphorus and total nitrogen sample analyses will be run in triplicate as part of the QA/QC protocol. CG volunteers will collect samples and flow data from these 30 stations for all sampled rain events and for all but one snowmelt runoff event. ISU will collect one set of field tile and storm sewer samples during spring snowmelt in order to determine discharge rates. ISU will provide training, sampling bottles, and equipment for CG volunteers.

3. Autosampler and flowmeter: One autosampler has been offered to this project by ISU, but it is necessary to equip this device with a flowmeter in order to collect flow-proportioned samples. Flow-proportioning of samples is necessary to determine timing of nutrient transport during rain events, and how the timing of grab-sample collections relates to nutrient concentrations in those samples.

4. Autosampler chemical analyses: The autosamplers will be calibrated to local conditions and programmed to take an average of twelve samples between each regular sampling event. Collected samples will be analyzed for total phosphorus, total nitrogen, and total suspended solids. Total phosphorus and total nitrogen sample analyses will be run in triplicate as part of the QA/QC protocol. ISU and CG will coordinate efforts concerning sample retrieval from autosamplers. Proposed locations for the autosamplers are sites 1 and 14 on the map. Site 1 is an urban storm sewer in a residential area with 12 curb intakes. Site 14 is a rural stream in an agricultural basin that combines tile intake sites 12 and 13, and tile outlet sites 15 and 16.

5. Possible human sewage impacts: CG will collect data from the County Sanitarian to identify homesteads with approved sanitation systems, and the local sewer and water service provider will be used to determine homesteads with and without city sewer services in the rural watershed. This data will be combined with drainage tile location information in order to determine suitable rural and unincorporated urban (Oakdale) sub-basins to sample for potential sewage flows into the lake. The upper limit to P input will be estimated by assessing maximum potential sewage release in the selected rural sub-basins. CG personnel will be used to survey the local rural population and determine the number of person-days present in the entire rural watershed. Potential human sewage inflows to Clear Lake will also be identified by testing selected inflows for chloride and caffeine on two dates during the course of this study. These two non-naturally occurring substances will be used as tracers to determine areas where human sewage inflows may be impacting Clear Lake. Known ratios of caffeine and chloride with nitrogen and phosphorus, as well as their relative inputs, in tile flow water.

6. Limnological monitoring program: Baseline limnological data are necessary at three determine sources of overall nutrient loads. Sampling stations will be established at three points in the lake (see map), and ISU will collect all lake samples. Samples will be collected at the water surface, 0.5 meter deep, and a 1 meter depth intervals to the lake bottom (maximum depth, 6m). A total of 20 samples will be collected from the 3 sites on 20 different sampling occasions. Samples will be collected between 0800 and 1600 hours. All samples will be analyzed for total phosphorus, total nitrogen, total suspended solids, temperature, and dissolved oxygen. Total phosphorus and total nitrogen sample analyses will be run in triplicate as part of the QA/QC protocol. Ammonia-nitrogen and unionized ammonia will be analyzed periodically, on appropriate dates. Samples collected from the upper mixing zone will be analyzed for chlorophyll a. Algal biomass in the upper mixing zone will be determined through algal genera identification, cell counts, and cell volumes, and reported in terms of biomass. Secchi disk transparency will also be determined at each sampling point.

7. New lake bathymetry: In order to accurately calculate nutrient budgets, a new bathymetric map showing depth contours will be constructed using differential GPS and digital, discriminating sonar. Sonar depths will be calibrated using a light-transmission, sediment surface detection array. Accuracy will be better than 20 cm vertical and 10 m horizontal. These data will be used to determine lake volume and water residence time in the lake, which are directly related to nutrient loads and budgets. Sediment thickness depth will be measured in "Little Clear Lake" (the western basin) and in Ventura Marsh using sediment probes.

8. Field trips: Field trips (20), meeting or presentation trips (5), and sample retrieval trips (17) to Clear Lake have been included in this proposal.

9. Basin characteristics, nutrients loads, and sources of impact: DNR has agreed, contingent upon contract approval, to contract the ISU GIS facility to provide a detailed description of the landuses in the lake watershed. Additionally, CG has proposed to have Vierbicher Associates (Madison, Wisc.) study the urban watershed, and Vierbicher has agreed to provide ISU with the hydraulic budget from storm sewers in urban areas. If those data become available within the time frame of this study, we will perform GIS analyses that will link landuse, topographic, and soils data with measured nutrient loads in streams, agricultural drainage tiles, storm sewer outflows, and in-lake measurements. This will be done to estimate the nonpoint loadings of nutrients and to identify portions of the watershed with the potential for producing the largest amounts of pollutants. The amount of nonpoint pollution loading produced by each landuse category including urban areas, will be estimated. These data, and data collected from lake samples, will be used in mass-balance equations to develop a nutrient budget for Clear Lake.

10. Final report preparation: During the course of this study, an Internet web site (WWW) will be established to post study information for public access. Representative digital images of dominant phytoplankton will be supplied for use in WWW sites and publicity. Once the pertinent information has been assembled and preliminary explorations have been made of the various alternatives for lake restoration, the investigators will assist CG and DNR in the conduct of a public hearing. Additionally, CG and DNR will receive digitized landuse data for the drainage basin in ARC/INFO format and a new bathymetric map of Clear Lake in paper and electronic form at the conclusion of this study.