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March 2010 (vol. 5, issue 1)
Groundwater Recharge in an Urbanizing Watershed: A Case Study
Contributed by Tim Erickson
Funded by Minnesota Pollution Control Agency
General hydrologic theory states that urbanization reduces groundwater recharge due to increased imperviousness. A case study in the Vermillion River watershed (VRW) shows that this might not always be the case. The Vermillion River is a naturally producing trout stream located in the southern metro area (Scott and Dakota counties) with a large portion (~80%) of its annual streamflow coming from groundwater sources.
Due to that high percentage, the Vermillion River should be susceptible and responsive to urbanization.
A recent streamflow analysis suggests that urbanization in the VRW has lead to no significant reduction in shallow groundwater recharge so far, and the trends may actually be towards a slight increase in groundwater recharge. The case study showed recharge from artificial sources, such as leakage for water distribution networks, excess crop and lawn irrigation, and septic system drainage, has replaced projected decreases in natural recharge from increased imperviousness.
Figure 1: Vermillion River Watershed, MN (Courtesy Vermillion River Watershed Joint Powers Organization)
Figure 1 shows a projected decrease in natural recharge of about 0.5 inches/yr over the period 1988 to 2005 due to increased imperviousness, yet artificial recharge has increased over the period, leaving the net recharge unchanged.
Figure 2: Annual groundwater recharge in the Vermillion River watershed (Erickson and Stefan, 2009a). (Courtesy T. Erickson)
Evidence of the additional recharge from man-made sources can be seen in the seasonal distribution of baseflow. Irrigation occurs only during the growing season. The distribution of baseflow shows increases from June through October, while precipitation actually increased only during June. Base flow is increasing during irrigation months, suggesting that groundwater recharge from irrigation is occurring.
Groundwater recharge has changed in the VRW, yet contrary to common assumption, total groundwater recharge in the VRW has not decreased. Natural recharge has been reduced, but has been replaced by recharge from “artificial” water sources. Further details of this study can be found in the reference section (Erickson and Stefan 2008, 2009a, 2009b).
Stormwater Treatment: Assessment and Maintenance Online Manual
Contributed by Andy Erickson
Funded by Minnesota Pollution Control Agency
The University of Minnesota and St. Anthony Falls Laboratory are
proud to announce the launch of a NEW online manual for the assessment and maintenance of stormwater treatment practices! The manual, “Stormwater Treatment: Assessment and Maintenance” builds on the knowledge and experience of engineers, researchers, faculty, consultants, watershed districts and many others. The purpose is to provide guidance and recommendations for measuring performance (i.e., assessment) and maintaining stormwater treatment practices.
The online manual provides methodology for assessment and maintenance through discussion, equations, examples, figures, and tables. The manual’s organization is centered around the four levels of assessment:
- Visual Inspection
- Capacity Testing
- Synthetic Runoff Testing
The online manual explains when to use each of the four levels and also provides detailed guidance on how to apply the four levels to many types of stormwater treatment practices.
One of the most powerful tools in the online manual is the guidance and supporting documents for visual inspection (level 1 assessment). Checklists for visual inspection have been developed from interactions with practitioners for many stormwater treatment practices. These standard checklists can be used to develop or augment an inspection program, document performance, and significantly reduce maintenance costs. In addition, capacity testing (level 2) can also be used to select and schedule specific maintenance tasks.
Some assessment programs will require accurate measurement or estimation of water budget components such as open channel flow, infiltration, and rainfall. The online manual provides recommendations for many measurement devices and techniques such as flow meters, weirs, rain gauges, infiltrometers, and permeameters.
Measuring performance for impacts on water quality will often require sampling and often analysis. The online manual provides detailed discussion and recommendations for sampling methods (e.g., in situ, grab, automatic) and analysis of water and soils, including quality assurance.
The online manual also provides detailed instructions with examples that illustrate how to calculate performance for a stormwater treatment practice based on load, event mean concentration (EMC), effluent probability, and percent exceedance. In addition, techniques and guidance for estimating uncertainty is also provided.
The manual also includes information on maintenance collected from public works departments and reports to provide guidance on expected maintenance schedules, level of effort, and expected costs.
The online manual includes case studies submitted by practitioners illustrating examples of assessment and maintenance using all four levels of assessment for various stormwater treatment practices. The manual is available online now at http://stormwaterbook.safl.umn.edu/
The online manual, “Stormwater Treatment: Assessment and Maintenance” is designed to provide guidance and recommendations for measuring performance (i.e., assessment) and maintaining stormwater treatment practices. The online manual makes this information available in a clear, concise, and easy-to-navigate format. Find it now at http://stormwaterbook.safl.umn.edu/
- Erickson, T.O., Stefan, H.G. 2008. Streamflow analysis on the Upper Vermillion River in Dakota County, Minnesota. Saint Anthony Falls Laboratory Project Report no. 507. University of Minnesota, Minneapolis, Minnesota
- Erickson, T.O., Stefan, H.G. 2009a Groundwater recharge in a coldwater stream watershed during urbanization. Saint Anthony Falls Laboratory Project Report no. 524. University of Minnesota, Minneapolis, Minnesota
- Erickson T.O., Stefan, H.G. 2009b. Projecting natural groundwater recharge response to urbanization in the Vermillion River watershed, Minnesota. Journal of Water Resources Planning and Management 135(6) 512-520