Retention Basins and Rain Gardens Retention Basins and Rain Gardens

A Possible Solution to Nutrient Pollution in

Natural Waters?

Population Growth & UrbanizationPopulation Growth & Urbanization

• Impervious surfaces lead to excessive storm water

• Because of urbanization hydrology is changed

Water Quality DeclineWater 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

DIRECT GROUND-WATER

DISCHARGE12%

DIRECT ATM. DEPOSITION

34%

SURFACE WATER

54%

Water Quality DeclineWater 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

Source: Hunchak-Kariouk and Nicholson, 2001

N Concentrations in Bay Streams, 1987-1997

Nitrogen Cycle – a Key PlayerNitrogen 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 PlayerNitrogen 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:&imgrefurl=http://www.h2ou.com/h2nitrogencycle.htm&docid=cnNVBqgcmbsX0M&imgurl=http://www.h2ou.com/h2images/NitrogenCycle-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=cl6dCdYpP-4HkM:&imgrefurl=http://landscapeforlife.org/give_back/3c.php&docid=KXqSzFlwUhex0M&imgurl=http://landscapeforlife.org/images/nitrogencycle.jpg&w=880&h=687&ei=qpGzT8b4C8Gbgwfk89T_Bg&zoom=1

You tube Video on Nitrogen CycleYou tube Video on Nitrogen Cycle

• Nitrogen Cycle Description

• 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

1994

/95

LA

ND

US

E/

LA

ND

CO

VE

R, %

INCREASING URBAN LAND

NO

3 C

ON

CE

NT

RA

TIO

N

(AS

N),

MG

/L

Source: Hunchak-Kariouk and Nicholson, 2001

Rationale

Nutrient Pollution StandardsNutrient 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

• 2008 National Estuarine Eutrophication Assessment suggests Barnegat Bay is in pretty bad shape

• Problems are expected to worsen over time

Rationale

Solution: Keep nutrients out of waterwaysSolution: Keep nutrients out of waterways

• Limit amounts of N- & P- containing 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 StudiesPrior 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 Bioretention hydrology & nutrient Removal in North CarolinaRemoval 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 Stormwater ponds & biofilters in Charles River, Boston MACharles 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 Traditional “Infiltration Style” Rain GardenGarden

• 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 Traditional “Infiltration Style” Rain GardenGarden

• 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 StudiesCurrent 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 GardensNitrogen-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

DisadvantagesDisadvantages

• More expensive

• New technology so not yet commonly constructed

• Replacement of vegetation every three years to ensure nitrogen removal capabilities are optimized

AdvantagesAdvantages

• 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?Questions?