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November 1997
by
John Cox, Environmental Specialist and Eric Livingston,
Environmental Administrator
Florida Department of Environmental Protection
Stormwater pollutants include a wide variety of substances
that accumulate on pervious and impervious surfaces between storms
and are transported by the next rainfall. These contaminants
include heavy metals, petroleum hydrocarbons, pesticides, and
many types of organic chemicals. Concern over the pollution potential
associated with urban runoff has led to unofficial requirements
that the residue deposited by these sources be taken to lined
landfills. An alternate proposed by some local governments is
use as clean fill and as a soil amendment similar to composted
domestic waste. Motivating factors include local initiatives
to reduce the volume of solid waste and recognition of desirable
characteristics such as the nutrient and organic content of these
materials. The result has been the imposition of extensive and
therefore expensive testing requirements for stormwater maintenance
entities seeking to comply with environmental concerns.
A study has recently been completed by DEP in cooperation with
14 FASU members throughout the state that compared samples of
stormwater sediment taken from 87 different treatment systems
with native soils in the area. The results of the report will
be presented at FASU's December Seminar in Orlando. The assessment
provides an evaluation of in place sediment, as well as the chemical
and physical properties of sediment and street surface contaminants
following removal and stock piling. The report enumerates and
lists the frequency of detection for fifty-three (53) pollutants
detected in concentrations greater than minimum detection limits.
Several problematic pollutants occurred from among approximately
168 organic and inorganic parameters. Contrary to expected results,
land use and BMP category were unreliable predictors and, not
well correlated with the percentage exceedance rate of screening
criteria.
METHODOLOGY
Previous studies have primarily involved characterization of in
place stormwater residuals subsequent to maintenance. Due to
time and budget constraints of the current study, it was important
to characterize materials by analysis of as few samples as possible.
Participants collected composite samples formulated from 3 to
7 individual aliquots at each site. A significant amount of best
professional judgment was necessary on the part of the local sampling
teams to ensure the collection of sample aliquots that would accurately
reflect the characteristics of the entire facility or stockpile.
As expected, the volume of stormwater sediment and associated
debris in these stockpiles was often extremely large or of a consistency
such that sample collection proved difficult.
Whenever possible, each participant collected a background sample
of native soil for analysis along with samples from stormwater
treatment facilities. Each background sample was analyzed for
the same array of parameters as the stormwater sediment for comparative
purposes.
The samples collected at each location were preserved on ice and
immediately sent to the FDEP Central Laboratory in Tallahassee
for analysis. The laboratory conducted approximately 24,000 analyses
during the course of the study.
Each participant in the study provided information concerning
the predominant land use class associated with the facilities.
The participants examined a total of eight land use classes.
The samples were also classified in terms of the BMP's or type
of maintenance operation. (The categories and number sites examined
are listed on page three.)
The allowable limits and screening level criteria associated with
various federal and state rules that may affect the disposal of
stormwater sediment was used to evaluate and rank each land use
and BMP category in terms the potential to be problematic for
stormwater facility operation and maintenance entities. Such
regulatory criteria included the Resource Conservation and Recovery
Act of 1976 (RCRA); Chapter 62-640, FAC, Domestic Wastewater Residuals;
Chapter 62-770, FAC, Petroleum Contamination Site Cleanup Criteria;
and, the Florida Preliminary Sediment Quality Assessment Guidelines.
RESULTS
All samples examined contained detectable levels of some constituents.
The background sites except for those submitted by the City of
Hialeah in Dade County contained lower levels and substantially
fewer contaminants than found in stormwater sediments. Six of
the fifteen background sites show values of some parameters above
soil cleanup criteria. As expected, most of the exceedances involve
detectable levels of Arsenic. A high value of 3.1 ppm was seen
in the Tallahassee area soils.
Roughly, only 10% of the test results exceeded the limits of detection.
The adjacent Table is a list of the pollutants found in detectable
concentrations at the 87 stormwater sediment sites examined.
Fifty-three (53) pollutants were found in concentrations that
exceeded minimum detection limits (MDLs). Several traffic related
metals (i.e., Cr, Pb, and Zn), were found at virtually all sites.
As a group, these inorganic pollutants were the most often detected
constituents in stormwater sediments.
Relative to organic contaminants, two pollutants were detected
at very high frequencies. Total recoverable petroleum hydrocarbon
(TRPH) levels exceeded MDL's at 90% of the test sites followed
by the pesticide Chlordane that was detected at an 82% frequency.
This pesticide was used extensively in years past for termite
control. Sale of the material has been banned since 1989 however,
it is known to be very persistent. Previously contaminated soil
transported and deposited onto streets via mud on vehicle tires
or wind and water erosion from lawns may account for contaminants
in street sweeper and catch basin debris.
Results Relative to Screening Level Criteria
Similar to results from our previous study, the Florida Sediment
Quality Assessment Guidelines (SQAG's) represent the most frequently
exceeded screening criteria. However, these (349) exceedances
represent only 1.4% of the total records. The heavy metal lead
was the most frequently detected inorganic pollutant in excess
of SQAG screening level criteria at 40% of the sites. However,
all seven metals evaluated exceeded TEL values on occasion. Copper,
Zinc and Cadmium were also shown to be somewhat problematic with
percentage exceedance rates of 22%, 14% and 11% respectively.
In contrast to results from our previous literature study, the
most problematic pollutant category found in stormwater sediments
relative to either SQAG's or Soil Cleanup Criteria were organic
contaminants. The pesticide Chlordane exceeded the TEL screening
level at all sites (82%) where detected. Most values (i.e., all
but 2) were in excess of probable effects levels (PELs). Likewise,
several traffic related PAH's were also shown to exceed SQAG criteria
more often than their inorganic counterparts. Pyrene led the way.
This compound was found in excess of TEL values at 61% of the
sites examined. Fluoranthene, Benzo a pyrene, Chrysene and Phenanthrene
followed closely with exceedance rates of 54%, 52%, 49% and 46%.
Stormwater Residuals Characteristics by BMP Type and Landuse
Classification
Similar to the 1995 study we also examined the data relative to
its origin in terms of BMP category and associated Landuse. Similar
to previous studies wet detention ponds and in-place sediments
in canals and lakes represented the facilities (BMP's) and sediment
maintenance categories of most interest to participants. Contrary
to the earlier study results the number of pollutants identified
and the average concentration is similar between these Landuse
and BMP categories. The exception would be a few landuse and maintenance
categories associated with systems such as sand filters, dry detention
ponds and vactor truck waste as well as high density residential
landuse that were sampled by only a few participants. These systems
show unexpected very low levels of pollutants as well as numbers
of constituents at detectable concentration in stormwater sediment.
The low pollution potential of these facilities is likely the
result of the extreme difference in sample number compared to
the other BMP categories, however.
CONCLUSIONS AND RECOMMENDATIONS
Existing data on the characteristics of stormwater sediments deposited
in stormwater systems throughout Florida is sparse and not easily
correlated due to differences in sampling methods used by the
various studies. The primary goal of this study was to collect
more sediment and debris samples from stormwater BMPs serving
various land uses from all parts of the state to increase our
database and get a better understanding of the need to test stormwater
sediments before disposal.
The result of the study has led to several interesting and to
some extent unexpected findings. Monitoring results do not show
any signs of significant differences by landuse category. However,
the large differences in monitoring frequency made a comparison
between landuse categories and BMP's difficult. Infrequently
sampled facilities produced highly variable results and caused
rankings to be extreme in both directions.
The question as to whether these materials warrant extreme concern
is more close to being answered. In most cases these residuals
are not "hazardous waste". However, the material is
contaminated well beyond levels associated with the raw stormwater
itself with a wide array inorganic and organic pollutants. Disposal
without proper precautions would not be recommended regardless
of the source of the residue.
Considering results of the previous literature survey conducted
by the Department and this project, the following recommendations
are offered relative to the disposal and testing of stormwater
generated residuals:
1. Stormwater sediments should be disposed
of by taking them to a permitted lined landfill and using them
for landfill cover. In general, such disposal should not require
characterization of the sediments except perhaps on an infrequent
basis. Sediment from BMPs serving industrial areas, fuel transfer
facilities and equipment maintenance yards where poor house keeping
and evidence of excessive contamination is obvious should be screened
for toxicity relative to RCRA criteria.
2. If it is preferable to dispose of stormwater sediments in an
unlined landfill, a borrow pit or via land application, then the
sediments need to be tested to assure that they meet the Clean
Soil Criteria specified in Chapter 62-770, FAC. Based on our
current database, concentrations of arsenic, lead and several
PAH's can be expected to exceed the Clean Soil Criteria.
3. More data is needed to develop guidelines on the frequency
of removing sediments from wet detention systems, especially when
considering the land use draining to the system.
4. While much maligned in most studies of BMP
treatment efficiency, the performance of catch basins and street
sweeping based on the strength of the residuals examined during
this study appears to be better than we would have projected.
However, the effectiveness of these controls, or the lack thereof,
is dependent on frequency of cleaning.
5. There currently are no state rules and programs that pertain
directly to the disposal of stormwater sediments. There is a
need to determine specific criteria to apply to stormwater sediments
when land application and use as clean fill are the preferred
use for these materials. |