Assessing Atrazine Input and Removal Processes in the Chesapeake

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Chapter 10

Assessing Atrazine Input and Removal Processes in the Chesapeake Bay Environment: An Overview 1,2

Haydee Salmun

and Kristin Goetchius

1

1

2

Department of Geography and Environmental Engineering, The Johns Hopkins University, Baltimore MD 21218 Current address: Department of Geography, Hunter College of the City University of New York, 695 Park Avenue, New York, NY 10021

This study focuses on the behavior of atrazine, a broad-spectrum herbicide extensively used in the Chesapeake Bay watershed in the cultivation of corn and sorghum. Studies show that atrazine can have detrimental effects on aquatic ecosystems. Reactions between atrazine and reduced sulfur species present in anoxic sediment porewaters may provide a significant sink for this agrochemical. A n integral approach involving field data analysis, laboratory studies and modeling is needed to understand the behavior of atrazine and to assess the toxic effects associated with the discharge of atrazine into the environment. As a first step, available data have been compiled to identify important inputs of atrazine and to estimate the resident mass of atrazine in the Chesapeake Bay. This paper presents a simple mass balance for the northern section of the Bay based on these data. This analysis illustrates that in order to assess and predict the long-term trends of atrazine for different loading scenarios, more comprehensive field data and more sophisticated models are needed that better capture the relevant physical and chemical processes.

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© 2002 American Chemical Society Lipnick et al.; Chemicals in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Introduction Estuaries are regions that support a wide variety of marine resources, including unique wildlife habitats and recreational opportunities. Because estuaries are dynamic zones of high biological productivity, most commercially important fish species spend a significant part of their life cycles i n estuaries. A t the same time, many estuaries are subject to nonpoint sources of pollution brought in via runoff. Organic contaminants originating from agricultural activities within the estuaries' watersheds are thus funneled directly into highly sensitive ecosystems. Even though many of these contaminants are herbicides with low mammalian toxicity, their effects at low concentrations on phytoplankton that are responsible for primary production in such areas are not well understood ( i ) . Herbicides have been implicated in the decline of submerged aquatic vegetation (SAV) in ecosystems such as the Chesapeake Bay (2). Furthermore, they often are highly toxic to fish and other aquatic fauna. A comprehensive review of the current knowledge and understanding of the behavior of pesticides in surface waters can be found in Larson et al (J). One of the most prominent groups of herbicides is the chloro-s-triazines, of which atrazine is the best-known example. Atrazine is a pre- and postemergence herbicide for the control of annual and perennial grass, as well as for the annual broad-leaved weeds, and is one of the fourteen organic compounds of potential concern in the Chesapeake Bay (3). In addition to atrazine itself, a variety of environmental degradation products may form, such as deethylated, deisopropylated and hydroxyatrazine. These dégradâtes may be more or less toxic than the parent compound, depending on chemical structure. For detailed studies of metabolites of atrazine and other herbicides in surface waters, see Meyer and Thurman (4). Many of the studies reported in the literature have been conducted in the laboratory or in artificial streams. Natural aquatic ecosystems are very complex, and it is difficult to find suitable controls that would aid in the assessment of the effects of atrazine in surface waters. It is the general consensus, however, that atrazine and its metabolites, i f present at sufficient levels, could exacerbate the chronically stressed Chesapeake Bay ecosystem. Atrazine levels are sufficient to inhibit various species of S A V in localized areas susceptible to agricultural runoff (5), so it does pose a threat in those areas. Significant concentrations of atrazine or persistent exposure to lower levels may result in changes in species composition and diversity, with species susceptible to atrazine being replaced by more resistant ones. For example, the less desirable water milfoil grass proved more resistant to atrazine than several other species of Bay grass that are important food sources for waterfowl (6). Atrazine can also affect aquatic fauna since it appears to be a potent environmental endocrine disrupter. A recent study based i n the U . K . investigated the effect of atrazine on the reproductive system and the consequent impact on the reproductive behavior of mature male Atlantic salmon (Salmo salar L) (7). It concluded that exposure

Lipnick et al.; Chemicals in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2002.


188 of the mature males to sub-lethal levels of atrazine in the water inhibited their ability to detect and respond to female priming pheromones. Atrazine is persistent in aquatic environments, as reflected by its high dissolved concentrations in tributaries to the Chesapeake Bay (8). It is estimated that about 1% of the total amount of atrazine applied to die Chesapeake Bay watershed reaches the aquatic environment (9, 10). A survey of studies of surface waters conducted between 1976-1993 found detectable atrazine concentrations in 67% of the analyzed samples (11). There was no indication of a decreasing trend of atrazine in surface water over a continuous span of 14 years during the study period despite a reported decrease in its use. Data on atrazine concentration collected by Hall et al (12) in 1995-1996 from the top meter of the water column during the high and low periods of atrazine use show that atrazine continues to be detected in all major and many secondary tributaries as well as in the Bay proper. The concentrations reported in the latter study appear to be smaller, although not significantiy. The persistence of atrazine in the Bay depends in part on the dynamics of mixing processes, as well as on chemical and biological reactions in the water column and sediment layers. Recent laboratory work suggests that atrazine may undergo transformation in the presence of naturally occurring inorganic sulfur nucleophiles (polysulfides) and that abiotic reactions with these reduced sulfur species may constitute a significant removal mechanism for atrazine in anoxic marine sediment porewaters (75,14). Effective management of these chemicals is required to minimize their potential adverse effects on living communities. To develop managerial and practical guidelines concerning their use in the watershed, it is necessary to study their transformation and transport from the application site, which can be effectively accomplished by an approach that uses a combination of field data and process modeling. This chapter provides an overview of the various hydrologie and chemical processes that occur in the Chesapeake Bay that may affect the fate and transport of atrazine. A summary of data on atrazine input to the Bay from different sources is presented, and a simple mass balance argument is formulated based on these data. The failure of the mass balance argument to provide a consistent picture of the fate of atrazine in the Bay demonstrates the need for more detailed data, complemented by a modeling approach that better captures the physics of transport, mixing, and chemical processes. A complete description of the processes that govern the fate of agrochemicals in the Bay is still beyond our current scientific knowledge. Simplified analyses are thus often used to gain some insight into the trends of chemical concentrations in the Bay. The next section presents a descriptive summary of the relevant physical and chemical processes occurring in the Bay. A compilation of data on atrazine inputs to the Bay is summarized in the following section. These data are used to estimate a resident atrazine mass, which is compared to estimations made from measured field values. We then


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present a simple mass balance for the Upper Chesapeake Bay region. This is followed by a brief discussion of the use of simple models in combination with field data to describe the behavior of atrazine in Bay waters and surrounding environment We conclude with a summary of the main points discussed in this overview.

Relevant Physical and Chemical Processes The Chesapeake Bay is the largest estuary in the United States. It drains a 164,000 square kilometer watershed with an extremely high ratio of land and population to volume of water (Figure 1). The distance between the mouth of the Bay to the mouth of the Susquehanna River is about 320 kilometers. The length of the shoreline is in the range of 7,000 kilometers long (75). The Bay is relatively narrow and shallow, with a mean width of approximately 15 kilometers and a mean depth of 10 meters, although the actual depth ranges from a few meters to about 30 meters in the deep channel. It holds about 18 trillion gallons of water and has a total surface area of about 8,000 square kilometers. The mean hydraulic residence time of the Bay is approximately 90 days (16). The overall circulation in the Bay is characterized by the flow of fresh water toward the ocean in the surface layer and the landward flow of saline water i n the deep layer arising from the entrainment of salty water at the head of the estuary, known as the gravitational circulation. The mean velocity associated with this circulation is approximately 0.1 m/s (77). The physical characteristics of the Chesapeake Bay are summarized in Table I.

Table L Physical Characteristics of the Chesapeake Bay Watershed area Distance from Susquehanna River to mouth of Bay Mean width Mean depth Maximum channel depth

164,000 k m 320 k m 15 k m 10 m

Surface area Upper Bay Surface Area Volume Hydraulic residence time Shoreline length

7,800 k m

2

30 m 2

2

1,480 k m 18 trillion gallons 90 days 7,000 k m



190

Figure 1. Chesapeake Bay and major tributaries (inset: Chesapeake Bay drainage basin). The Bay is classified according to circulation and stratification patterns as a partially mixed estuary (18). Mixing for the most part is due to turbulent motion caused primarily by tidal action and secondarily by climate conditions. To assess the impact agrochemicals could have on the well-being of estuarine ecosystems and to determine the role estuaries may play in effecting contaminant removal, an integral approach that includes the relevant hydrologie and chemical processes must be considered. Agrochemicals such as the chlorinated triazine herbicides tend to undergo abiotic hydrolysis relatively slowly; for example; the half-life for the uncatalyzed hydrolysis of atrazine is 1800 years at p H 6.97 and 25°C (19). They are at best slowly degraded by



191 microbes, which are present at low concentrations typical of aquatic environments (20, 21, 22). Moreover, they tend to be of low volatility, and sufficiently hydrophilic that they are not efficiently removed from the water column via scavenging by settling particles (23). Peak agrochemical loading often occurs in the spring, coinciding with seasonal anoxia during which sulfate (abundant in sea water) may undergo dissimilatory microbial reduction to generate reactive sulfur nucleophiles. Concentrations of polysulfides, which are particularly reactive sulfur nucleophiles, can attain higher values in the sediment porewaters. In order for atrazine that is introduced into fresh waters to react with polysulfides, it must first mix down the water column to reach the sulfidic sediment porewaters. This complicates attempts to evaluate the significance of reactions with sulfur nucleophiles based simply on a comparison of reaction half-lives with overall hydraulic retention times; rates of vertical mixing also play a key role and must be included in modeling approaches.

Atrazine in the Chesapeake Bay Systematic investigation of pesticides in the Chesapeake Bay watershed began in the 1970's in response to the observed decline in S A V and fish populations during that period. Investigations typically spanned a few years and tested for specific families of herbicides (e.g., chloro-s-triazines and chloroacetanilides) and insecticides (e.g., organophosphates and chlorinated hydrocarbons) in various media. Detection limits and consistency in the methods of data collection improved with time. The data that are summarized below consist of measurements of concentrations of atrazine in the inputs to surface waters at several locations throughout the Bay. We should stress that data are sparse; spatial and temporal distributions have to be estimated and/or extrapolated from measurements in the top 0.5-1.0 m of the surface layer.

Inputs of Atrazine The three major pathways for atrazine to enter the Chesapeake Bay are surface runoff, groundwater inflow, and wet and dry atmospheric deposition. Processes such as hydraulic flushing, air-water transfer and chemical reactions will then influence the final concentration and distribution of atrazine in the Bay.

Surface Runoff There are few studies on the watershed scale that estimate the atrazine loading to the Bay due to agricultural activity. Input of atrazine and other



192 herbicides, such as metolachlor, from surface runoff primarily occurs in the months of May through August, as demonstrated in Figure 2. In the following analysis it is assumed that all of the surface runoff enters the Bay via river flow. Three tributaries, the Susquehanna, the Potomac, and the James Rivers account for approximately 80 to 85 percent of the fresh water flow into the Bay from the northern and western regions. These three tributaries also dominate herbicide input to the Bay (8). Foster and Lippa (8) present the most comprehensive data sets available. They estimated that 2,700 kg of atrazine were loaded into the Bay in the period 1992-1993: 1,700 kg/yr via the Susquehanna, 780 kg/yr via the Potomac, and 220 kg/yr via the James. Similar estimates of atrazine loading were reported by the U . S. Environmental Protection Agency (USEPA) (15, 24) for the Susquehanna and the James Rivers. Godfrey et al. (25) showed that the relative error in the estimation method used by Foster and Lippa (8) was less than 40%. The Chester and Choptank Rivers are the major pathways of atrazine input from the Eastern Shore of the Bay. Based on concentration and flow rate, these rivers account for a combined mass load of approximately 100 kg/yr. The total input of atrazine via surface runoff is thus 2,800 kg/yr.

Figure 2. Flow rate and atrazine and metolachlor concentrations in the Susquehanna River in 1994 (26).


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Groundwater To estimate the amount of atrazine input via groundwater inflow, both concentration and groundwater seepage measurements are needed from areas surrounding the estuary. Studies conducted within the Bay's watershed in the 1980s and early 1990s measured concentrations of atrazine i n groundwater ranging between 0.3 to 3 pg/L (27, 28, 29) throughout the year. To estimate atrazine loading due to groundwater inflow we assume that a representative atrazine concentration of approximately 1 pg/L exists in the groundwater that discharges into the Bay. Although atrazine concentrations vary seasonally depending on time of application and type of bedrock, this estimate is a conservative one and is taken to characterize atrazine concentration in groundwater throughout the year. Fewer studies are available on groundwater seepage rates due to the experimental difficulty associated with such measurement. Reay et al. (30) reported an average groundwater discharge into the Bay i n the southern region of the Eastern Shore of 0.35 L / ( m hr), which corresponds to a flow velocity of 0.0084 m/day. They also observed that discharge into the Bay occurs predominantly within a 50-meter distance offshore. Assuming that the estimated shore length is 7,000 km and neglecting the bottom-slope effects over this distance, the bottom area for inflow is estimated as 350 k m (50 m χ 7,000 km). Consequently, the estimated atrazine groundwater loading to the whole Bay, computed as groundwater flux χ discharge concentration χ bottom area, is 1,100 kg/yr. 2

2

Atmospheric Deposition Despite the low Henry's law coefficient for atrazine, it has been detected in rainfall in agricultural regions in the U.§. (31, 32). Goolsby et al. (32) estimated atrazine deposition rates that range between 11 and 60 pg/m /yr in the northeastern region of the U.S. W u (31) estimated the wet deposition at Rhode River Watershed (MD) as 102 pg/m /yr in 1977 and as 9.7 pg/m /yr in 1978. U S E P A (15) estimated the combined wet and dry deposition of atrazine to the Bay as 770 kg/yr. Recent studies by Harman-Fetcho et al (33) estimated the wet deposition of atrazine per rain event over a study period from April 17-June 26, 1995. Data from twenty four events were used to estimate that the total wet deposition flux of atrazine in Solomons, M D was 6,400 ng/m over the entire study period, ranging from 5 to 1,300 ng/m per event. If these measurements can be considered characteristic for the entire Bay, they would result in a load of approximately 50 kg of atrazine over the 70 days reported for the study period, which is comparable to the lower bounds reported in the literature (31, 32). Here we use 102 pg/m /yr as an upper limit for the amount of atrazine introduced to 2

2

2

2

2

2


194 the whole Bay resulting in an atmospheric deposition load of approximately 800 kg/yr, which agrees very well with the estimate by the U S E P A .


Total Inputs The total amount of atrazine entering the whole Bay via the various pathways is 4,700 kg/yr. The estimates are summarized in Table II. The National Oceanic and Atmospheric Administration ( N O A A ) reports that approximately 481,582 kg (1,061,707 lb) of atrazine is applied to the Chesapeake Bay drainage area annually (11). Using this value and assuming that the atrazine loading is 1% of that mass, approximately 4,800 kg/yr of atrazine are washed into the Bay. This value closely agrees with the one presented here. Atrazine inputs to the Bay derive mainly from: (1) surface runoff, estimated to contribute about 60% of the total load, (2) groundwater flux, contributing about 23% of total atrazine load, and (3) atmospheric input: wet and dry deposition, which constitutes 17% of the atrazine load to the Bay. Table II. Summary of Atrazine Inputs to the Chesapeake Bay Entire Bay

Upper Bay

source

kg/yr

%

kg/yr

%

Surface Runoff

2,800

60

1,800

84

Groundwater

1,100

23

200

9

Atmospheric

800

17

150

7

TOTAL

4,700

100

2,150

100

Resident Atrazine Mass in the Bay To date, only sporadic temporal and spatial measurements exist for atrazine concentration in the surface waters of the Chesapeake Bay, and the quality of the data only allows for an order-of-magnitude estimate of the resident atrazine mass. To obtain this estimate, we assume that the bulk of this herbicide resides in the water column and the sediment layer, with negligible amounts in biota and the surface microlayer, a layer of up to 1 mm thick at the air-water interface. Samples for atrazine concentration in surface waters are typically taken from the top 0.5 m in the Bay. Reported concentrations for the data collected during 1976-1993 range between 0.03-4.3 pg/L, and show no evidence of spatial or temporal trends (34). Data histograms (not shown here) yield a log-normal



195 distribution with a mean and variance of 0.7 pg/L and 0.99 pg/L, respectively. Applying this mean concentration to a 6-m thick, well-mixed surface layer, and multiplying by the surface area in Table I, we can compute the average mass of atrazine in the surface layer. The average mass for the Bay is 32,760 kg (0125,890 kg, 95% confidence interval), of which 6,224 kg (0-23,919 kg, 95% confidence interval) is assumed to be in the northern 110 km of the Bay. For reference, we note that these values are of the same order as those reported by Schottler and Eisenreich (35) for the atrazine inventory in, for example, Lake Michigan of the Great Lakes. The lack of a standard protocol for sediment sampling introduces additional variability in reported concentrations to the natural variability encountered in the Bay. K r o l l and Murphy (34) detected atrazine in 93% of their sediment samples taken in 1978 at a Maryland site, with a maximum measured concentration of 2.15 ppb. Boynton et al. (36) measured atrazine concentrations below 1 ppb i n the sediment layers in the eastern and western tributaries of the Bay in 1980. More recently, Eskin etal. (37) reported no detectable atrazine concentrations in 40 stations covering the main stem and tributaries of the Bay. The difference between this work and earlier studies may reflect a removal of atrazine from the sediment layer, or, simply, better accuracy and reliability in the analytical methods. It is reasonable to adopt the most recent results and to consider the atrazine mass resident in the sediment layer insignificant.

A Simple Mass Balance for the Upper Chesapeake Bay A large fraction of atrazine is introduced to the Bay via surface runoff in the northern section. The largest runoff contribution to the Chesapeake Bay is the Susquehanna River, which discharges directly into the Upper Bay. In addition, the Chester and Choptank Rivers discharge into the northern section of the Bay. A larger amount of data on atrazine concentrations is available for this region resulting from a larger network of stations. Consequently, a first step toward understanding atrazine behavior in surface waters and sediment porewaters of the entire Bay is to understand its behavior in the northern section and we focus on this region in the remaining of the present discussion. The Upper Bay extends approximately 110 km south of the outlet of the Susquehanna River (Figure 1). Its mean depth ranges from 12 m over the first 60 km to 25 m in the remaining 50 km. As for the entire Bay, total input and output can be computed for the Upper Bay to provide a simple budget analysis, and the results compared to resident mass that was calculated from measured concentrations. The Susquehanna River fluvial input of atrazine in the Upper Bay region is 1,700 kg/yr (8). The Chester and Choptank Rivers discharge approximately 100 kg/yr. Both contributions yield a total surface flux of 1,800 kg/yr into this region. To estimate the amount of atrazine introduced as groundwater inflow,


196 we assume that the loading to the northern region of the Bay via this pathway is proportional to the ratio of its surface area (1.48xl0 m ) to that of the whole Bay (7.8xl0 m ), which is 0.19. Thus, the input in groundwater to the upper region is 200 kg/yr. In a similar fashion, from estimates for the entire Bay we estimate that the atmospheric deposition to the Upper Bay is 150 kg/yr. In summary, a total of 2,150 kg/yr is loaded to the Upper Bay with surface runoff estimated to contribute about 60% of the total load, groundwater flow contributing about 23% of total atrazine load, and atmospheric input constituting 17% of the atrazine load. These results are summarized in Table II. Measured atrazine concentration range between 0.03 and 4.3 pg/L, with a mean value of 0.7 pg/L. The total surface area of the Upper Bay is 1.48 χ 10 m and i f we assume that atrazine is reasonably well distributed over the top 6 m (the surface mixed layer) then the volume over which atrazine concentration is quasi-uniform is approximately 9 χ 10 m . We note that considering only the first meter of the water column, where data from measurements are reported, does not change this simple mass balance argument significanfly. With these assumptions (consistent with the data as reported in the literature) the total mass of atrazine resident in this part of the Bay ranges from 270 k g for the lowest reported concentrations to 38,700 kg for the upper limit. The latter is highly unrealistic. If, on the other hand, we use the mean concentration value of 0.7 pg/L, we obtain a total mass of about 6,000 kg. The outflow from the Upper Bay over a depth of 6 m is approximately 11,000 m /s (24). The rate of mass of atrazine removed by this flow is obtained by multiplying the outflow value times the concentration value. The lowest concentration value (0.03 pg/L), i f assumed constant throughout the year, results in about 10,400 kg/yr removal of atrazine from this portion of the Bay, or 5,200 kg/yr i f that concentration is present only half the year. Yet, we can only account for 2,150 kg of atrazine as input, hence this simple mass balance argument indicates that either a substantial source of atrazine is unaccounted for or, more likely, that concentrations are lower and are not uniform in time and space. Furthermore, atrazine must be leaving this region in the top layer by some diflusion-like mechanism. To achieve steady state, atrazine concentrations in the outflow surface water need to be 0.006 pg/L, but there is no evidence that supports this estimate. More and better data and models are clearly needed to elucidate some of these important issues. 7


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Modeling Atrazine Behavior in the Chesapeake Bay Recent work suggests that rates of atrazine reaction with reduced sulfur species present in the sediment porewaters may be sufficiently rapid to provide an important removal mechanism in sulfate-reducing environments (13). The most significant reaction of atrazine with reduced sulfur nucleophiles occurs with polysulfides. This reaction can be modeled as a pseudo first order reaction


197 assuming a polysulfides concentration range in the sediment porewaters of 0.020.2 mM (38) and a second order rate constant of 5.6 χ 10" M' s' (14). The multi-box model known as MAS AS (Modeling Anthropogenic Substances in Aquatic Systems developed by Ulrich, 1991) was used in the hope that simulations with this approach could guide the development of more realistic models to study this potential removal mechanism. We assumed an idealized estuary with some basic aspects of the characteristics of the Chesapeake Bay, such as gravitational circulation andriverflow. Three types of chemical reactions were incorporated in the model: photolysis, hydrolysis, and abiotic reaction with reduced sulfur species in the sediment layer. Simulations revealed that, although reduced sulfur species in the sediment have the potential for degrading atrazine, stratification and reduced mixing across the pycnocline minimize the importance of such reactions. Simulated atrazine concentrations in the Bay showed that it tends to accumulate in the upper water layers, even during the weak stratification in fall period. A large discrepancy was observed between simulated and measured atrazine concentrations in surface waters, with measured concentrations being one order of magnitude higher than simulated values. Although MASAS has been used successfully to describe atrazine behavior in Swiss lakes (39, 40), our results confirm the limitations of this type of model for use in large estuaries. These models assume that the rates of vertical mixing are smaller than reaction rates and that both are much smaller than horizontal mixing rates. This assumption is not valid in most estuaries. In particular, horizontal advection and horizontal diffusion in the Chesapeake Bay are comparable while vertical diffusion is a fast process that acts over short distances, and a model must account for all three. In this environment, atrazine that is discharged to the surface waters could be horizontally distributed over a distance of 1 km over a period of one week, since the time scale of horizontal advection-diffiision processes is 10 -10 s (approximately 3 hours). As atrazine is distributed horizontally, it also mixes vertically down the water column. With the estimates of vertical diffusivity for the Bay that are available in the literature, for a depth of 10-20 m the time scale for vertical diffusion processes is on the order of 15 minutes, and can be as short as 3 minutes. The sulfidic waters are in the sediment porewaters and atrazine needs to be transported to the water-sediment interface in order to encounter and react with reduced sulfur species. The characteristic horizontal and vertical scales that describe the flow in the Bay indicate that it is possible for atrazine to reach the depth of the water-sediment interface before it is horizontally transported out of the system. The subsequent exchange at the water-sediment interface depends on many factors, including half-life of atrazine, the hydraulic residence time of the bottom layer, turbulent processes, and other characteristics of the water column above the sediment layer. Simple box models cannot capture the dynamics necessary to describe these exchanges that ultimately govern the fate of atrazine in the Bay.


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198 The Bay environment includes many fresh, brackish and salt water marshes and tidal flats which cover an area of approximately 1,700 square kilometers. These marshes are present along both the Eastern and Western Shores of the Chesapeake Bay and constitute an important component of the Bay ecosystem, in part because they are highly reactive environments and may play an important role in removing pesticides before they reach the main estuary. If these removal processes result in the detoxification of contaminants, they might be considered for use in treating agricultural runoff. Salt marshes are shallow systems, with a water column depth that ranges from less than 0.2 m to 0.6 m (41). They can be tidally flooded for long periods, although the degree of inundation depends on the geographic location and season. During periods of flooding salt marshes are characterized as anoxic environments, and high concentrations of reduced sulfur species have been detected (42). Because of their shallowness, anoxic characteristics, and the likely presence of atrazine in salt marshes, there exists a potential for abiotic reactions between atrazine and polysulfides. Atrazine can reach a salt marsh via groundwater flux, atmospheric deposition, tidal flow, and surface runoff. In a shallow water column atrazine reaches the sediment porewaters where it can react with polysulfides, which have been detected in sediments of several salt marshes in the area, including the Great Marsh in Delaware (42). The polysulfides concentration in sediment porewaters in salt marshes of the Bay region is reported to be as high as 0.3 m M (43). Studies have also shown that concentrations of polysulfides in salt marshes vary seasonally (42\ with concentrations peaking during periods of anoxia. During spring and summer, salt marsh vegetation injects oxygen into the upper sediment layer (44\ which may partially oxidize hydrogen sulfides and bisulfides resulting in the formation of polysulfides (45). Therefore, peaks in polysulfides concentration are likely to coincide with the peaks in atrazine concentration, making spring and summer the most probable seasons to observe the reaction between them. To investigate the fate of atrazine in salt marshes, and the potential role these marshes play i n affecting the concentration of atrazine in the Bay proper, modeling studies and more detailed data are needed.

Summary and Conclusion A n overview of the behavior of atrazine, an extensively used corn herbicide, in the Chesapeake Bay environment has been presented. Atrazine has a differential effect on varying species of submerged grasses and could shift species composition in areas where it reaches the Bay or tributaries in large quantities. There is concern that it can become an added stressor to an environment already heavily affected by excess nutrients, rapid changes in land use and population growth. There is also concern about the effects of various



199 atrazine metabolites that may have an increased or reduced toxicity depending on their chemical structure. The Chesapeake environment includes the presence of salt marshes, which may play a role in transfonning agrochemicals. Sail marshes are very complex systems and there is very little or no data readily available on atrazine behavior in marshes surrounding the Bay. There is some limited information on atrazine concentrations for the Bay itself and some tributaries. Some of the physical processes that must be included in models oi salt marshes are also relevant to the estuary. From our investigation we conclude that available data on atrazine, although not comprehensive, constitute an adequate basis for modeling studies and allow for preliminary estimates of the fate of atrazine in the Chesapeake Bay. However, a great limitation on these studies resides in the lack of observations throughout the water column. Box models that have successfully been used to study atrazine behavior in lakes cannot capture the basic processes that govern transport in estuaries as complex as the Bay. To look at different loading scenarios and to attempt to predict trends, we need a multi-dimensional convective-diffiisive model with chemical processes included. We stress here again that a great limitation on initializing and validating such models resides in the lack of observations of atrazine distribution with depth in the water column. We anticipate that the study of atrazine input and removal mechanisms in a salt marsh will be hampered by a similar lack of detailed data and the appropriate hydrodynamic model to be used with them.

Acknowledgements The first author gratefully acknowledges the assistance of Y. Farhan with the initial compilation of the data. Discussions with A. Lynn Roberts and Katrice Lippa helped us understand the issues associated with the relevant chemical reactions. We also thank the anonymous reviewers for their comments and suggestions. This research has been partially funded by the US Environmental Protection Agency through STAR Grant R826269-01-0. The contents of this manuscript do not necessarily reflect the official views of the USEPA, and no official endorsement should be inferred.

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