Stormwater
pollution has historically been assessed by measuring the concentration of
pollutants in the water column. For example, the pollutant-removal performance
of structural best management practices (BMPs) has been extensively
characterized using flow-weighted pollutant concentrations (EMCs) in inlet and
outlet water column samples. Over the last few years, it has been recognized
that “gross pollutants” such as larger-size sediment, organic debris, manmade
debris, and grass and leaves could also be significant sources of pollutants.
The recently completed “ASCE Guideline for Monitoring Stormwater Gross Solids”
(EWRI 2009) gives recommendations for monitoring and testing of the gross solids
components to determine their pollutant content.
With
total maximum daily loads (TMDLs) being set for nutrients in many Florida
waterbodies, interest has been created in preventing nutrients bound in organic
materials from entering waterbodies. What is lacking is a method to quantify the
nutrient reduction benefit from BMPs that trap gross
solids.
Previous
Studies
The
rate at which nutrients would leach from a mixture of green grass and dried oak
leaves when submerged in water over extended time was examined by Strynchuk,
Royal, and England (2001). The purpose of the testing was to simulate conditions
in which leaves and grass were trapped in a water-filled vault box such as a
baffle box or continuous deflective separation (CDS) unit. Known masses of the
grass and leaf solids were placed into batch leaching bottles filled with water
from a drainage canal and incubated for up to 180 days in a dark building open
to the air. Samples of the solid material and leachate were periodically removed
and analyzed for total Kjeldahl nitrogen (TKN), total oxidized nitrogen, total
nitrogen (TN), total phosphorus (TP), biochemical oxygen demand (BOD), and
chlorophyll a.
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Photo: Otterbine Barebo |
Figures
1 and
2 show how TKN and TP concentrations fluctuated with time for the solids
and liquid phases. In Figure 1,
TKN was reduced in solids in the first 24 hours, with a corresponding increase
in liquid TKN. In the first 30 days, levels of TKN fluctuated and then
stabilized at about the initial dry level as the nitrification cycle progressed.
Figure 2 shows a drop in the first day for solids TP levels and an increase in
leachate TP levels from 1.9 to 1,100 g/kg. After the first day, TP
concentrations stabilized and remained fairly constant for the remainder of the
testing period. These figures indicated that organic debris stored in a
water-filled BMP would rapidly release small amounts of TKN and large amounts of
TP to the water, which could be subsequently be flushed out with the next
rainfall event. The conclusion of the report was that use of water-filled BMPs
resulted in little nutrient-removal benefit from trapping and storing organic
debris in a wet state.
Testing
oak leaves, McCann and Michael (1998) showed similar results of rapid release of
nutrients in water.
Objective
The
objective of this study was to examine the potential of dry storage to reduce
the leaching of nitrogen and phosphorus from grass clippings. The hypothesis was
that storing green grass clippings in a dry state would enable a
nutrient-reducing decomposition process to occur. The hypothesis was tested
using fresh clippings of St. Augustine and Bahia grass collected from four
locations in Brevard County, FL. The grass clippings were initially
characterized for wet and dry mass, TN, TKN, nitrogen oxide (NOx), and TP;
allowed to dry for 30 days; and retested for the same
parameters.
Methods
A
sampling and testing regime was developed to simulate an inlet trap BMP that
would filter grass clippings washed into an inlet and allow the grass clippings
to dry before being removed from the inlet. Inlet traps are typically cleaned
two to three times annually, but seldom at intervals of less than 30 days. A
30-day drying time was used in this study.
Two
samples of St. Augustine grass and two samples of Bahia grass were collected by
a landscaping contractor in Brevard County on a random basis from locations he
maintained (Table 1). For each of the two grass types, one sample was taken from
a location where the grass had never been fertilized and one sample was taken
from a nearby location where the grass had been fertilized and irrigated
regularly. Fertilization of the two samples took place on a semiannual basis
with 6-14-8 fertilizer.
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| Photo: Otterbine Barebo |
All
grass samples were taken on July 31, 2008, during Florida’s wet season. The most
recent antecedent rainfall occurred on July 24, 2008. At each sample location,
two samples of freshly cut green grass from a lawnmower bag were taken from
different parts of the site. The two samples from each location were thoroughly
mixed and washed with tap water to simulate a rainfall that washed grass
clippings into an inlet. For each of the four combined samples, a subsample was
removed, placed on ice, and sent to a National Environmental Laboratory
Accreditation Conference (NELAC)–accredited analytical
lab.
The
samples were analyzed for solids, water, and nutrient content. Test procedures
for solid organic tissues, rather than standard aqueous test procedures, were
used as shown in Table 2. Nutrient concentrations in grass samples were reported
on a dry
weight basis.
The
remaining grass materials were kept in separate open baskets in a warm, dry,
dark building for 30 days and allowed to dry with no more exposure to water,
simulating being trapped in an underground inlet filter that had no subsequent
rain events, which would leach out nutrients. Each of the subsamples was then
sent to the laboratory for a second analysis.
Results
Laboratory
results are shown in Table 2. Fresh-cut green grass had an average TN
concentration of 18,250 mg/kg, while air-dried grass had an average TN
concentration of 4,173 mg/kg. The drying process for the four grass samples gave
TN reductions ranging from 58% to 96%, with an average 80% removal (Figure 3).
TN is the sum of TKN and NOx. TKN levels were consistently orders of magnitude
higher than NOx levels, with TN and TKN values being the same in most
cases.
Figure
4 shows that TP reductions from the drying process were not as high, nor as
variable as TN reductions. Initial TP average concentrations were 3,175 mg/kg.
After drying 30 days, the average TP concentration dropped to 2,118 mg/kg. TP
reduction ranged from 23.1% to 49%, with an average reduction of 35%.
Conclusions
In
both Bahia and St. Augustine grass samples, air-drying resulted in significant
reductions of TN concentrations and moderate reductions in TP concentrations.
This study provides a starting point for estimating the reduction of nutrient
loading to receiving waters that can be achieved by BMPs that capture grass in
stormwater runoff, or by ordinances that prevent grass clippings from being
deposited onto roads and into storm drains. Therefore, air-drying of grass
samples in a manner that simulates dry storage in an inlet trap BMP serves as a
type of “treatment process” that reduces nutrient masses reaching receiving
waters.
BMPs
such as inlet traps that filter grass clippings and keep them dry will allow the
treatment process of decomposition to occur and should be more effective at
reducing nutrient loads from gross organic solids than BMPs that trap organic
debris and keep them in a water-filled chamber.
A
practical method of using the information from this report would be to track the
mass of grass clippings collected in inlet traps and compute the annual mass of
grass removed. Obtaining a lab analysis of representative grass samples to
determine TN and TP concentrations would give end condition concentrations and
masses. Working backward with the above reduction factors would give beginning
condition masses, which would be the mass of nutrients removed from the system.