Surface and Groundwater Interactions. Introduction

Transcription

1 Surface and Groundwater Interactions Robert Pitt Department of Civil, Construction and Environmental Engineering University of Alabama Tuscaloosa, AL Introduction Infiltration of stormwater has become an increasingly important stormwater management tool. Attempts are made to restore pre-development water balance, by increasing infiltration to balance increased paved surfaces. This has been the leading solution to protect the aquatic habitat in surface waters. Infiltration also important in areas having combined sewers to reduce frequency and magnitude of overflows (CSOs). 1

2 Introduction (cont.) Scattered information is available addressing groundwater impacts in urban areas. Major information sources include: Historically known high chlorides under northern cities EPA 1983 NURP work on groundwater beneath Fresno and Long Island infiltration basins NRC 1994 report on groundwater recharge using waters of impaired quality USGS work on groundwater near stormwater management devices in Florida and Long Island A number of communities throughout the world (including Portland, OR; Phoenix, AZ; WI; Tokyo; plus areas in France, Denmark, Sweden, Switzerland, and Germany, etc.) Research Elements/Methodology Our research on stormwater and groundwater interactions began during an EPA cooperative agreement to identify and control stormwater toxicants. Our first efforts were based on extensive literature reviews for reported groundwater data beneath urban areas and management options. Initial stormwater - groundwater impact report published by EPA (1994) and Lewis Publishers, CRC Press (1996). Have since continued to investigate pollutant fates in amended and natural soils and filtration media. 2

3 Areas of Concern that Determined Groundwater Contamination Potential (weak-link model): Presence of constituent in stormwater (function of flow phase and source area) Mobility of constituent in vadose zone (function of soil and constituent properties) Treatability of constituent (mostly a function of constituent association with particulates and infiltration device design) Stormwater Infiltration Controls in Urban Areas Roof drain disconnections Bioretention areas Rain gardens and amended soils Porous pavement and paver blocks Grass swales and infiltration trenches Percolation ponds Dry/injection wells 3

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7 Recent Research Investigating Potential Groundwater and Stormwater Interactions Grass swales (different grasses and soils, and amended soils) Infiltration rates through disturbed urban soils (greatly affected by compaction) Soil amendments to improve infiltration in urban soils and to remove stormwater pollutants Column studies to test different material for amendments Large-scale and micro-scale water mass balances at locations using infiltration as a stormwater control 7

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10 Laboratory and Field Verification Tests Conducting high resolution, low-head laboratory tests with soils, each at three levels of compaction. Conducting field measurements of soil density and infiltration rates. These field soils are then brought to the laboratory for moisture and texture analyses, in addition, laboratory infiltration tests will be conducted on these soils at the different compaction conditions. 10

11 Rain Garden Designed for Complete Infiltration of Roof Runoff Effects of Compost- Amendments on Runoff Properties Another portion of the EPA-funded research was conducted by Dr. Rob Harrison, of the University of Washington They examined the benefits of adding large amounts of compost to glacial till soils at the time of land development 11

12 Water Quality and Quantity Effects of Amending Urban Soils with Compost Surface runoff rates and volumes decreased by five to ten times after amending the soils with compost, compared to unamended sites. Unfortunately, the concentrations of many pollutants increased in surface runoff from amended soils, especially nutrients which were leached from the fresh compost. However, the several year old test sites had less, but still elevated concentrations, compared to unamended soil only test plots. 12

13 Amended Soil Compared to Unamended Soil Constituent Surface Runoff Mass Discharges Runoff Volume Phosphate Ammonia Nitrate Copper Zinc Subsurface Flow Mass Discharges Results Sources of pollutants were monitored Classes of stormwater constituents that may adversely affect groundwater quality: Nutrients Pesticides Other organics Microorganisms Metals Salts 13

14 Nutrients Nitrates are one of the most frequently encountered contaminants in groundwater, mostly in agricultural areas and where septic tanks are used (very mobile, but relatively low stormwater concentrations). Phosphorus contamination of groundwater has not been as widespread, or as severe, as that of nitrogen compounds (less mobile, but in higher concentrations in stormwater). Heavy Metals Studies of recharge basins receiving large metal loads found that most of the heavy metals are removed by sedimentation, or in the first few inches of soil. Order of attenuation in the vadose zone from infiltrating stormwater is: zinc (most mobile)>lead>cadmium>magnesium>copper> iron>chromium>nickel>aluminum (least mobile) 14

15 Heavy Metal Concentrations ( g/l) Observed in Sheetflows from Source Areas Metal Highest Observed Source Area Concentration Cadmium 220 Street runoff Chromium 510 Roof runoff Copper 1250 Street runoff Lead 330 Storage area runoff Nickel 130 Landscaped area runoff Zinc 1580 Roof runoff Dissolved Fraction of Heavy Metals (fraction passing 0.45 m filter) Heavy Metal Filterable Fraction (%) Cadmium 20 to 50 % Chromium <10 % Copper <20 % Iron Small amount Lead <20 % Nickel Small amount Zinc >50 % 15

16 Pesticides The greatest pesticide mobility occurs in areas with coarse-grained or sandy soils, without a hardpan layer. Pesticides decompose in soil and water, but the total decomposition time can range from days to years. Pesticide mobility can be retarded or enhanced depending on soil conditions (Henry s Law and soil adsorption constants). Stormwater Organic Compounds Found in Stormwater Most commonly occurring organic compounds found in urban groundwaters include phthalate esters and phenolic compounds. Groundwater contaminants from organics occurs more readily in areas with sandy soils and where the water table is near the land surface. 16

17 Occurrence of Stormwater Organic Compounds know to Adversely Affect Groundwaters Compound High conc. ( g/l) Detect. Freq. (%) Significant sources Pyrene Oils, gasoline, bitumen, coal tar, wood preservatives Chlordane Insecticide Butyl benzyl phthalate Plasticizer Bis (2-chloroethyl) ether Fumigant, solvents, insecticides, paints, varnishes Bis (2-chloroisopropyl) ether Pesticide manufacturing 1,3-Dichlorobenzene Pesticide manufacturing Compound Other Common Stormwater Organic Compounds (PAHs) Highest conc. ( g/l) Detect. Freq. (%) Significant sources Benzo (a) anthracene Gasoline, wood preservative Benzo (b) fluoranthene Gasoline, motor oils Benzo (k) fluoranthene Gasoline, bitumen, oils Benzo (a) pyrene Asphalt, gasoline, oils Fluoranthene Oils, gasoline, wood preservative Naphthalene Coal tar, gasoline, insecticide Phenanthrene Oils, gasoline, coal tar 17

18 Filterable Fractions of Stormwater Organic Compounds Organic Compound Benzo (a) anthracene Bis (2-chloroethyl) ether Bis (2-chloroisopropyl) ether 1,3-Dichlorobenzene 75 Fluoranthene 65 Naphthalene 25 Filterable Fraction (<0.45 m) None observed in filtered fraction Irregular Irregular Phenanthrene None observed in filtered fraction Pyrene 95 Chlordane None observed in filtered fraction Butyl benzyl phthalate Irregular Microorganisms Viruses have been detected in groundwater where stormwater recharge basins were located short distances above the aquifer. Factors affecting survival of bacterial and viruses in soil include ph, antagonism, moisture, temperature, sunlight, and organic matter. The major bacterial removal mechanisms in soil are straining at the soil surface and at intergrain contacts, sedimentation, sorption by soil particles, and inactivation. 18

19 Salts Sodium and chloride travel down through the vadose zone to the groundwater with little attenuation. Studies of depth of penetration in soil have shown that sulfate and potassium concentrations decrease with depth, whereas sodium, calcium, bicarbonate, and chloride concentrations increase with depth. 19

20 Potential Problem Pollutants were Identified Based on a Weak-Link Model Having the Following Elements: Their abundance in stormwater, Their mobility through the unsaturated zone above the groundwater, and Their treatability before discharge. Constituent Example Weak-Link Model Influencing Factors Abundance in Stormwater Mobility (sandy/low organic soils) Nitrates low/moderate mobile high Filterable Fraction (treatability Chlordane moderate intermediate very low Anthracene low intermediate moderate Pyrene high intermediate high Lead moderate very low very low 20

21 Links Depend on Infiltration Method (contamination potential is the lowest rating of the influencing factors) Surface infiltration with no pretreatment (grass swales or roof disconnections) Mobility and abundance most critical Surface infiltration with sedimentation pretreatment (treatment train: percolation pond after wet detention pond) Mobility, abundance, and treatability all important Subsurface injection with minimal pretreatment (infiltration trench in parking lot or dry well) Abundance most critical Example Applications: Low Abundance Abundance is important for all cases, therefore if a constituent is in low abundance in stormwater, the groundwater contamination potential will always be low, irrespective of infiltration method. Examples for most areas include: 2-4-D, VOCs, anthracene, napthalene, and cadmium; some areas may have higher concentrations of these constituents, with an increased contamination potential. 21

22 Example Applications: No Pretreatment Before Infiltration Through Surface Soils (grass swale example) Mobility also important If a compound is mobile, but in low abundance (such as for nitrates), the contamination potential is low. If compound is mobile and also in high abundance (such as chlorides in cold regions that use salt de-icers), the contamination potential would be high. Example Applications: Sedimentation Pretreatment Before Infiltration (treatment train) All three factors important Chlordane would have low contamination potential with sedimentation pretreatment (because much of the chlordane would be removed), even though it has moderate abundance and intermediate mobility. If no pretreatment, the chlordane contamination potential would be moderate. 22

23 Moderate to High Contamination Potential Surface Infiltration with no Pretreatment Lindane, chlordane Surface Infiltration after Sedimentation Injection after Minimal Pretreatment Lindane, chlordane Benzo (a) anthracene, bis (2-ethylhexl phthalate), fluoranthene, pentachlorophenol, phenanthrene, pyrene Fluoranthene, pyrene 1,3-dichlorobenzene, benzo (a) anthracene, bis (2-ethylhexl phthalate), fluoranthene, pentachlorophenol, phenanthrene, pyrene Enteroviruses Enteroviruses Enteroviruses, some bacteria and protozoa Nickel, chromium, lead, zinc Chloride Chloride Chloride Recommendations to Reduce Groundwater Contamination Potential when using Infiltration in Urban Areas Combined sewer overflows should be diverted from infiltration devices because of poor water quality. Snowmelt runoff should be diverted from infiltration devices because of high concentrations of salts. Construction site runoff must be diverted from infiltration devices due to rapid clogging. 23

24 Recommendations to Reduce Groundwater Contamination Potential when using Infiltration in Urban Areas (cont.) Infiltration devices should not be used in most industrial areas. Runoff from critical source areas (mostly in commercial areas) need to receive adequate pretreatment prior to infiltration. Runoff from residential areas (the largest component of urban runoff in most cities) is generally the least polluted and should be considered for infiltration. 24

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27 Recommended Stormwater Monitoring to Evaluate Potential Groundwater Contamination Most past stormwater quality monitoring efforts have not adequately evaluated stormwater s potential for contaminating groundwater. Urban runoff contaminates with the potential to adversely affect groundwater: Nutrients (especially nitrates) Salts (especially chlorides) VOCs Pathogens Bromide and TOC (if considering disinfection) Pesticides Other organics Heavy metals Conclusions Most of the stormwater toxic organics and metals are associated with the nonfilterable fraction, and are easiest to remove using conventional sedimentation practices. Pollutants in filterable forms have a greater potential of affecting groundwater. Sorption and ion exchange mechanisms can be used to capture filterable toxicants. These can be enhanced by amending soils in the infiltration area, or by using media filtration as pretreatment. Treatment trains having multiple components and processes (especially sedimentation and infiltration) offer good solutions in most areas. 27

28 References and Acknowledgements Pitt, R., S. Clark, R. Field, and K. Parmer. Groundwater Contamination from Stormwater. ISBN Ann Arbor Press/CRC. 219 pages Pitt, R., B. Robertson, P. Barron, A. Ayyoubi, and S. Clark. Stormwater Treatment at Critical Areas: The Multi-Chambered Treatment Train (MCTT). U.S. EPA, Wet Weather Flow Management Program. EPA/600/R-99/ pgs. March Clark, S. and R. Pitt. Evaluation of Filtration Media for Stormwater Treatment. U.S. EPA, Water Supply and Water Resources Division. EPA/600/R-00/016, 442 pgs. October Pitt, R., J. Lantrip, R. Harrison, C. Henry, and D. Hue. Infiltration through Disturbed Urban Soils and Compost- Amended Soil Effects on Runoff Quality and Quantity. U.S. EPA, Water Supply and Water Resources Division. EPA 600/R- 00/ pgs. December