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Retention Basins and Rain Gardens A Possible Solution to Nutrient Pollution in Natural Waters? Population Growth & Urbanization • Impervious surfaces lead to excessive storm water • Because of urbanization hydrology is changed Water Quality Decline • Influx of pollutants – Nitrogen (N2) and phosphorous (P) • Excessive quantities can cause eutrophication – ↑ in algae – ↓ dissolved O2 levels – Leads to widespread fish kills Water Quality Decline • Key problem is nutrient pollution, particularly nitrogen (N) pollution • About ½ half of N load to Barnegat Bay comes from surface runoff (Kennish et al. 2007) • Removing nutrients from surface water before it enters the Bay is cost effective way to improve Bay health DIRECT ATM. DEPOSITION 34% DIRECT GROUNDWATER DISCHARGE 12% Source: Hunchak-Kariouk and Nicholson, 2001 SURFACE WATER 54% N Concentrations in Bay Streams, 1987-1997 Nitrogen Cycle – a Key Player • N2 makes up over 75% of air – Contained in Amino Acids, proteins & nucleic acids of living things • Comes in various forms – Only ammonium (NH4+) & nitrate(NO3-) are useable by plants & algae Nitrogen Cycle – a Key Player • Nitrogen-fixation – anaerobic bacteria convert atmospheric into NH4+ & NO3- • Vital part of Nitrogen Cycle – – – – Made of several parts Nitrification Denitrification Ammonification http://www.google.com/imgres?q=nitrogen+cycle&hl=en&gbv=2&biw=1024&bih=566&tbm=isch&tbnid=Oer5J0d51BtSrM:&img refurl=http://www.h2ou.com/h2nitrogencycle.htm&docid=cnNVBqgcmbsX0M&imgurl=http://www.h2ou.com/h2images/Nitrog enCycle-lgr-F.jpg&w=697&h=605&ei=qZKzT9DaBNCN6QHmobiQCQ&zoom=1 http://www.google.com/imgres?q=nitrogen+fixation&hl=en&biw=1024&bih=566&gbv=2&tbm=isch&tbnid=cl6dCdYpP4HkM:&imgrefurl=http://landscapeforlife.org/give_back/3c.php&docid=KXqSzFlwUhex0M&imgurl=http://landscapeforlife.org/images/nit rogencycle.jpg&w=880&h=687&ei=qpGzT8b4C8Gbgwfk89T_Bg&zoom=1 You tube Video on Nitrogen Cycle • Nitrogen Cycle Description Rationale • Human development increases impervious cover in watersheds • Traditional rain gardens collect storm water from impervious areas and infiltrate it through soil, slowing delivery to rivers and bays, but don’t reliably remove nutrients, esp. Nitrogen Source: Hunchak-Kariouk and Nicholson, 2001 INCREASING URBAN Nutrient Pollution Standards • US Federal Government changed standards in 2007 • Resulted in a call to lower nutrient pollution from industry, commercial, and residential areas • ↓ non-point pollutants – ↓ N2 and P Rationale • 2008 National Estuarine Eutrophication Assessment suggests Barnegat Bay is in pretty bad shape • Problems are expected to worsen over time Solution: Keep nutrients out of waterways • Limit amounts of N- & Pcontaining nutrients making their way into aquatic ecosystems • In turn, algal blooms would decrease • Also slow flow and possibly divert storm water away from rivers and streams • Reduce downstream flooding Prior Studies • Many studies have investigated ways to reduce nutrient pollution in waters of East Coast of United States e.g. – Charles River in Boston – North Carolina • Aimed at decreasing excessive nitrogen and phosphorous in bodies of water Bioretention hydrology & nutrient Removal in North Carolina • Bioretentions built in proximity to Shopping Center with proper filtration can allow in upwards of 40% of nitrogen & phosphorous removal • Water outflow also reduced, especially in warmer seasons Stormwater ponds & biofilters in Charles River, Boston MA • Reduction of phosphorous from nearby industries into bodies of water can be achieved • Up to 65% removal with the use of detention pond and filter system Traditional “Infiltration Style” Rain Garden • Designed to slow water flow • Absorbs excess water flow from nearby impervious surfaces • Flood reduction by in increased absorbency • Decreases soil erosion Traditional “Infiltration Style” Rain Garden • Unreliable at nutrient removal • Maximum phosphorus and nitrogen removal is between 30 – 40% • Inadequate at reducing the amount of nutrients making its way into bodies of water • Water generally flows vertically through traditional style rain garden Current Studies • Univ. of New Hampshire Stormwater Center is a leader in study of methods for nutrient removal • Their proposed solution uses horizontal filtration of water through a “Gravel Rain Garden” • Dense plantings at surface means lots of plants to take up nutrients through their root system • More important, though, is the anaerobic environment created in crushed stone that lies beneath. • Allows for denitrification Nitrogen-Removing Rain Gardens • Create extensive saturated zone designed to maintain anoxic conditions to promote denitrification through use of impervious liner • Longer retention of water, increases nitrogen removal efficiency • Base of gravel to provide high surface area for microbial activity • Top with soil and plant with typical rain garden plants Disadvantages • More expensive • New technology so not yet commonly constructed • Replacement of vegetation every three years to ensure nitrogen removal capabilities are optimized Advantages • Existing ponds and detention basins can be turned into subsurface gravel rain gardens • Works well in cold climates • Much superior and more reliable nutrient removal than obtained with traditional “infiltration style” rain gardens Questions?