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   Effects of Rain Gardens on the Quality of Water in the Minneapolis-St. Paul Metropolitan Area of Minnesota, 2002-04%VAPOTRANSPIRATION,YSIMETER7ELL)NFLOW SAMPLER,YSIMETER/VERFLOW SAMPLER7ELL2AIN GARDEN4EMPORARY PONDING)NFILTRATION7ATER TABLE'ROUND WATER.OT TO SCALEFigure 1.Schematic diagram of expected processes and monitoring points of a rain garden.garden through storm drainage and subsequently allowed topond temporarily until water can infiltrate into the ground and(or) be taken up by the garden vegetation. Rain gardens aredesigned to overflow during large runoff events with a specified recurrence interval. Retained water undergoes a varietyof biotic or abiotic processes that include uptake by vegetationwith subsequent transpiration, evaporation, and infiltration.Sedimentation and biological transformation also removesuspended solids, nutrients, and other contaminants that couldbe detrimental to receiving waters.Purpose and ScopeThis report discusses the results from sampling rain-garden sites located throughout the Minneapolis-St. Paul metropolitan area during June 2002 through October 2004. Theamount of precipitation, the volume of inflow and overflow,and the amount of infiltration and other losses were not measured and were not considered in this report. Sources of waterto the rain gardens were evaluated only qualitatively becausemost sites had relatively flat contributing areas that may havesporadically added to the inflow.Sampling focused primarily on determining the concentration of a few selected constituents considered to be indicative of runoff including suspended solids (particulate materialcarried in suspension by flowing water, also called residue,total at 105 degrees Celsius, suspended), nitrogen, phospho-rus, chloride, and gross measures of dissolved constituents.Although the changes in mass transported throughout thesystem relative to sources were not measured, the data provideinitial information to evaluate measured concentrations incomponents of the water system in a rain garden that weresampled and determine how they interrelate at each of thesites sampled. Other important factors including precipitation characteristics, antecedent conditions, and flow volumesfrom which to compute loads were not measured. Longduration storms that exceeded the capacity of the automaticsamplers were not adequately sampled. Other factors thatcould contribute to a better understanding of the systemsincluding delineation of drainage areas, contributing drainageareas, and detailed information on land-use characteristics alsowere beyond the scope of this study. Determining how thesesamples relate to the existing climate or changing climate,changes in land use, and other factors also was beyond thescope of this report.Results in this report will improve understanding aboutthe fate of chemical constituents transported to rain gardensin runoff and can be used by water managers and land-useplanners to better understand the environmental impacts andeffectiveness of rain gardens in protecting water quality. Thiscan lead to improved designs and enhanced protection forsurface and ground water. Findings about the significance oflocal site conditions, such as soil texture and permeability, willallow for improved rain-garden design that more effectively

Introduction   treats storm-water runoff. Results should be transferable toother areas of the nation.AcknowledgmentsThe USGS gratefully acknowledges the MetropolitanCouncil of the Twin Cities, Department of EnvironmentalServices, who supported and helped design this study. Gratitude also is extended to all of the local units of government,developers, and landowners who assisted with installationand allowed continued access to these sites. The followingcompanies provided maps of rain-garden design that wereused to sketch simplified diagrams of sites shown in figures 3,4, 5, and 7, respectively: Bonestroo, Rosene, Anderlick, andAssociates; Emmons and Olivier Resources, Inc.; WestwoodProfessional Resources; and Barr Engineering.Description of Study SitesThe Minneapolis-St. Paul metropolitan area of Minnesotais situated on relatively flat to gently rolling land that wasmostly prairie before settlement. Relief is much greater nearthe rivers and streams that cross the area. Most of the area hasan altitude of about 1,000 ft or less. The rain-garden sites thatwere studied range in altitude from about 925 to more than1,000 ft.Five rain-garden sampling sites were selected withthe input from the Metropolitan Council of EnvironmentalServices and other interested stakeholders. The sites represent a wide range of hydrologic and land-use conditions thatalso represent a range of impervious surface conditions thatincluded parking lots, driveways, walkways, and roofs. Therain gardens also may receive runoff from grassy areas including athletic fields and lawns.The five sites (fig. 2) are located in the communities ofChanhassen, Hugo, Lakeville, Minnetonka, and Woodbury.The sites are distributed across an area of nearly 4,000 mi2within the seven-county Minneapolis–St. Paul metropolitanarea. The percentage of clay in the uppermost 5 ft of the soilprofile (fig. 2) was estimated from the State Soil Geographic(STATSGO) data for Minnesota by averaging the percentageof clay in mapped units of the soil profile. The STATSGO dataset is “ a digital general soil association map developed bythe National Cooperative Soil Survey. It consists of a broadbased inventory of soils areas that occur in a repeatable patternon the landscape and that can be cartographically shown atthe scale mapped. The soil maps for STATSGO are compiledby generalizing more detailed soil survey maps. Where moredetailed soil survey maps are not available, data on geology,topography, vegetation, and climate are assembled, togetherwith Land Remote Sensing Satellite (LANDSAT) images.Soils of like areas are studied, and the probable classificationand extent of the soils are determined.” (U.S. Department ofAgriculture, 2005).The sampling sites at each of the rain gardens sampledfor this study are listed in table 1. Also included is information, such as site identifier, that would be useful in locatingadditional information about these sites and the actual datathat are currently (2005) available.The climate across the area ranges from relatively warmerin the southwest to relatively cooler in the northeast. However, these long-term climate patterns often are confounded byweather systems that have local effects.Climate conditions in the area are relatively uniform.Normal mean monthly temperatures at the Minneapolis -St.Paul International Airport (1971-2000) vary from 70.6 F(August) to 13.1 F (January) and average 45.4 F annually.These normal means are slightly higher in the south and westand lower in the north and east. Inner-city areas are slightlywarmer than outlying areas at all times of the year (MinnesotaDepartment of Natural Resources, 2005a).Seasonal variability in precipitation also occurs in thearea. Nearly two-thirds of normal mean annual precipitationfalls during the growing season from May through Septemberand only 8 percent of the normal mean annual precipitationfalls in the winter (December through February). Normalannual precipitation varies across the area from about 29 to30 in. and increases from the southwest to the northeast. Thelowest normal mean monthly precipitation occurs in August(Minnesota Department of Natural Resources, 2005b).During parts of this study, precipitation was less than normally would be expected. This resulted in collection of fewersamples than were anticipated during certain seasons.Climatic variations are minor among the rain-gardensites. Weather variations resulting from convective thunderstorms are more likely to create variability among sites andresult in substantial differences in the amount of precipitationdelivered to the rain gardens studied. Use of other hydrologicdata, including records of precipitation, inflow and overflowvolumes, and rates of evapotranspiration, which could havebeen useful to estimate loading and attenuation of materialsdelivered to the rain gardens, was beyond the scope of thisstudy.The effectiveness of rain gardens is related to the topographic and land-use settings of each site, including the textureand hydraulic conductivity of soils and unconsolidated glacialdeposits that underlie the sites. Most of the rain-garden siteswere chosen with the intention of capturing runoff immediately downstream from an impervious surface, such as aroof or parking lot, and soil characteristics were not alwaysas important a consideration as location. Although the soiltypes (fig. 2) are a general guide, local variations may occurthat cannot be mapped at the scale shown. Soil tests and otherengineering considerations similar to a percolation test takeinto account the variables that contribute to infiltration in arain garden. The site at Lakeville apparently had a highlypermeable substrate that resulted in little or no overflow fromthe storms sampled, but another rain garden being installed(2005) about 1 mi north of the existing rain garden does nothave the permeable substrate and requires special engineering

   Effects of Rain Gardens on the Quality of Water in the Minneapolis-St. Paul Metropolitan Area of Minnesota, 2002-0493 30'93 45'93 15'93 00'92 45'r ISANTIveRiNorth &SouthCHISAGO CenterLakesRumSHERBURNE10ChisagoLake35ANOKA45 15'94PelicanLakeWRIGHTpiFridleyMaple GroveBrooklyn nt PaulaMF.MinnetonkaSaint r10121244 45'n16910SpringLakeApple leoes94Richfield494S.Eden PrairieCARVERMaplewoodRiverCrowetninLakeMC towCrF.Coon RapidsBrooklyn Park35WLakeN.CrowBlaineipSt.issBuffaloLake45 RiCroix94 IBLEY52erRiv35nonCan44 30'Lake ByllasbyGOODHUELE SUEURRICENICOLLETBase from U.S. Geological SurveyDigital data, 1:100,000, 1985U.S. Albers projectionEXPLANATIONPERCENT CLAY00201010203030404050 MILES50 KILOMETERS0 to less than 88 to less than 1616 to less than 2424 to less than 32MINNESOTAMap areaRAIN-GARDEN STUDY SITEFigure 2.nesota.Location of rain-garden sampling sites and percentage of clay in soils in the Minneapolis-St. Paul metropolitan area of Min-

Introduction   Table 1. Rain-garden sites sampled in the Minneapolis - St. Paul metropolitan area of Minnesota[Latitude and longitude: DD, degrees; MM, minutes; SS, seconds; NA, not applicable]Site identifierSite nameLatitudeDDMMSSLongitudeDDMMSSStart dateEnd date445149093365502Rain garden lysimeter near Chanhassen, MN4451490933655Aug. 2003Nov. 2004445149093365503Rain garden inflow near Chanhassen, MN4451490933655Sep. 2003Sep. 2004445150093365402Rain garden background lysimeter near Chanhassen, MN4451500933654Sep. 2003Oct. 2004445150093365403Rain garden outflow near Chanhassen, MN4451500933654Aug. 2003Sep. 2004450943092593901Rain garden well at Hugo, MN4509430925939Aug. 2002Nov. 2004450943092593902Rain garden lysimeter at Hugo, MN4509430925939Aug. 2002Nov. 2004450943092593903Rain garden inflow at Hugo, MN4509430925939Jul. 2002Sep. 2004450946092593901Rain garden background well at Hugo, MN4509460925939Jun. 2002Nov. 2004450946092593902Rain garden background lysimeter at Hugo, MN4509460925939Aug. 2002Nov. 2004450946092593903Rain garden outflow at Hugo, MN4509460925939NANA443914093171801Rain garden well at Lakeville, MN4439140931718Sep. 2002Nov. 2004443914093171802Rain garden lysimeter at Lakeville, MN4439140931718Sep. 2002Nov. 2004443914093171803Rain garden inflow at Lakeville, MN4439140931718Sep. 2003Sep. 2004443920093173501Rain garden background well at Lakeville, MN4439200931735Oct. 2002Nov. 2004443914093173602Rain garden background lysimeter at Lakeville, MN4439140931736Sep. 2002Nov. 2004443920093173503Rain garden outflow at Lakeville, MN4439200931735NANA445643093253801Rain garden well near Minnetonka, MN4456430932538Nov. 2003Oct. 2004445643093253802Rain garden lysimeter near Minnetonka, MN4456430932538Aug. 2003Oct. 2004445643093253803Rain garden inflow near Minnetonka, MN4456430932538Aug. 2003Oct. 2004445645093254001Rain garden background well near Minnetonka, MN4456450932540Aug. 2003Oct. 2004445645093254002Rain garden background lysimeter near Minnetonka, MN4456430932538Aug. 2003Aug. 2004445645093254003Rain garden outflow near Minnetonka, MN4456450932540Aug. 2003Oct. 2004445512092564401Rain garden well near Woodbury, MN4455120925644Oct. 2002Oct. 2004445512092564402Rain garden lysimeter near Woodbury, MN4455120925644Aug. 2003Oct. 2004445512092564403Rain garden inflow near Woodbury, MN4455120925644Jun. 2003Sep. 2004445516092563801Rain garden background well near Woodbury, MN4455160925638Oct. 2002Oct. 2004445516092563802Rain garden background lysimeter near Woodbury, MN4455160925638Aug. 2003Aug. 2004445516092563803Rain garden outflow near Woodbury, MN4455160925638Jun. 2003Sep. 2004

   Effects of Rain Gardens on the Quality of Water in the Minneapolis-St. Paul Metropolitan Area of Minnesota, 2002-04to encourage infiltration (Keith Nelson, Lakeville City Engineer, oral commun., 2005). Soils maps indicate that both theestablished and under-construction rain gardens are situated inthe same type of soils. Other considerations such as the watertable altitude also may affect infiltration and may need to beevaluated on a site-specific basis.The closest resemblance to a soil test available for thisstudy was the lithologic logs for the monitoring wells thatwere installed at the rain-garden sites (table 2). Materialsencountered ranged from coarse sand to clay, with some graveland cobbles. The least-permeable material was present atChanhassen, where wells were not installed because the claywas impervious to water transport.Many of the rain gardens have multiple inflows becausewater can come from several surrounding impervious surfaces. Rain gardens generally are situated in low-lying areas.Because it is difficult to install multiple intakes that can collect a representative, proportionate sample from each of theinflows, one inflow was selected at each site that was expectedto provide the largest, most representative inflow to that raingarden. These largest inflows are assumed to be the appropriate sampling sites for this qualitative study.The typical site installation (fig. 1) encompassed twoautomatic samplers. One was configured to collect water at theprimary site of inflow. The other was configured to samplethe overflow when sufficient water passed into and throughthe rain garden to generate overflow. A well and lysimeterwere installed within the rain garden to measure the quality ofwater that might infiltrate from the water ponded within therain garden. A well and lysimeter also were installed in anarea believed to represent background conditions that are notinfluenced by infiltration from the rain garden.The Chanhassen site (fig. 3) is located within the parkinglot of the University of Minnesota Landscape Arboretum. Thesite is underlain by clay-rich soils derived from glacial till.Consequently, observation wells were not installed. Inflowconsisted of runoff from a series of parking lots interspersedwith vegetated strips. The inflow was sampled by using afloat-activated sampler at the primary point where storm-waterrunoff enters the rain garden. Overflow samples from thissite are unique because they come from a drain tile installedbeneath the rain garden. The drain tile empties into a culvertthat serves as an overflow during major rainfall events. Datacollection from this site was complicated by several factors.Arboretum staff attempted to maintain flowering plants in therain garden that required frequent watering, which could produce overflow without inflow. The source of this added waterwas assumed to be the public water supply for the arboretum.In addition, ongoing construction surrounded the site duringthe sampling period. The effects of construction on runoffloading to the rain garden were not measured during the study.The Hugo site (fig. 4) is underlain by sand and gravelthat is part of the Anoka Sand Plain. Runoff to the rain gardenis from the parking lot and from the roof of Hugo City Hall.Other nearby areas also may contribute runoff to the raingarden. Ground water is assumed to flow toward the site fromthe northwest, an area that consists mostly of gravel roads andathletic fields. The background well and lysimeter are locatedin this area. No overflow from the rain garden was observedduring the study and consequently an overflow sample was notcollec

ABSTRACT Rain gardens are a popular method of managing runoff while attempting to provide aesthetic and environmental ben-efits. Five rain-garden sites in the Minneapolis - Saint Paul