Tracking Recent DDT Contamination in a Northern New England Watershed

Ecological research since 2005 into potential causes of declines in loon population at Squam Lake, New Hampshire, U.S.A., revealed multiple potential causes, but no particular source of contaminants. In 2017, tributary sediment analyses revealed specic sub-watersheds transporting contaminants to the lake (Vogel, 2019). For this study, from 2018 to 2020, we used an approach to this problem that allowed for rapid source area determination of DDT using soil and sediment analyses. We nd modern presence of p,p’ isomers of DDT and DDE within the Bennett Brook sub-watershed, arising from 60-year-old orchard applications and a former barn. Highest concentrations, 723 μg/kg p,p’ DDT and 721 μg/kg p,p’ DDE, occur near the barn’s foundation rubble. DDT exceeds that of the daughter product, DDE, in some of the sub-watershed’s soils, including but not limited to the barn site. In the soils where DDT>DDE, we infer delayed breakdown of DDT. DDT<DDE occurs in the streambed and lake deposits, as well as some soils. A Pb-210 dated sediment core, collected near the outlet of Bennet Brook, shows continuous accumulation of the daughter products, DDE and DDD, from 1951 to the present. Residuals are derived from multiple sources within the sub-watershed, including orchard soils, the barn site, and sediment accumulations in the stream. These DDT residues fall below mandatory soil remediation levels for the State of New Hampshire, but exceed some sediment quality guidelines for protection of aquatic life. Bioaccumulation of p,p’ DDE is evident in craysh that reside in Bennett Brook.


Introduction
A popular pesticide, DDT was banned in the United States in 1972, due to its adverse impacts on ecosystems and persistence in the environment (USEPA, 1975). Although the ban began almost 50 years ago, DDT is still detected in biota, sediments, and soils throughout the world (Rissato et al., 2006;Andrews et al., 2009;Hu et al., 2010;Yang et al., 2013;Bettinetti et al., 2016;Kang et al., 2016;Feingold and Benoit, 2018;Khuman et al., 2019;Das et al., 2020). In 2004 to 2005, 44% of all resident loons died on Squam Lake, NH, U.S.A., followed by ongoing low reproductive successes (Vogel, 2010). Tests of loon eggshells revealed elevated levels of DDT residues, along with other contaminants including PCBs, PFOS, PBDEs, dioxins and furans (Vogel, 2010). Sediment samples collected in 2015 and 2016 from 14 tributary inlets to Squam Lake revealed elevated levels of DDT residues at the mouth of Bennett Brook (Vogel, 2017). Sediment transport is the leading mechanism for environmental DDT dispersal, and sediments are a major sink for DDT residues (USDOI, 1998;Yang et al., 2013). DDT is a non-polar pesticide that does not easily dissolve in water but readily sorbs to organic matter, clay and silts in soils and sediments by hydrophobic bonding (Boul, 1995;Leatherman, 1997;Wohl, 2015). DDT often concentrates in the uppermost soil layers. which can be eroded by heavy rains and oods. Once transported to rivers and lakes, DDT becomes bioavailable to aquatic organisms. DDT readily dissolves, and is stored, in fatty tissues, resulting in bioaccumulation. Adverse impacts in piscivorous birds highest on the aquatic food chain include eggshell thinning, reproductive impairment, and low edging success (Ames, 1966;Nygård, 1983;USDOI, 1998).
DDT breaks down into DDE and DDD under aerobic and anaerobic conditions, respectively (Boul, 1995;Quensen et al., 1998;Gao et al., 2013). Therefore, in the upper, aerobic layers of soils, DDE is the main residue, whereas in anoxic lake bottoms and wetlands, DDD is the main residue and the most easily microbially degraded (Pham et al., 1993;Boul 1995). DDT is composed of two isomers of about 85% p,p' DDT and 15% o,p' DDT (ATSDR, 2002). The isomer p,p' DDT degrades into p,p ' DDD and/or p,p' DDE, and o,p' DDT degrades into o,p' DDD and/or o,p' DDE (Ricking and Schwarzbauer, 2012). The half-life of DDT is variable, generally 2 to 35 years depending on local environmental conditions (Nash and Woolson, 1967;Lichtenstein et al., 1971;Dimond and Owen, 1996;Leatherman, 1997). The rate of degradation may vary continuously, often with faster rates immediately after application or introduction into the environment (Lichtenstein et al., 1971). The half-life in soil varies with soil type, organic carbon content, soil pH, temperature, susceptibility to volatilization, microbial activity, tilling, and ooding (Nash and Woolson, 1967;Pham et al., 1993;Boul et al., 1994;Boul, 1995;Dimond and Owen, 1996;USDOI, 1998). Since the ban of DDT in 1972, the ratio of DDE and DDD to DDT has increased in soils, sediments, and biota (USEPA, 2017). However, some recent studies report higher concentrations of DDT relative to its breakdown products in sediments and soils (Johnson et al., 1988;Pham et al., 1993;Muir et al., 1995;Bailey et al., 2005;Hu et al., 2010). The persistence of DDT and its breakdown products makes the pesticide a long-term threat to aquatic life.
The aim of this study is to better understand the fate and impact of DDT, including transportation pathways of DDT residues from soils to aquatic sediments to aquatic life via an array of processes. Although DDT has ultimate impacts on ecosystems, the pathways for introduction to the ecosystem begin with geosystem and sedimentary processes. This interdisciplinary systems approach guides the objectives of this study, which are to: 1) identify source(s) of DDT residues in an affected watershed; 2) determine the duration and triggers for mobilization of the contaminant; and 3) determine if DDT from the Bennett Brook watershed is entering the aquatic food chain.

Study area
Squam Lake, New Hampshire (NH), U.S., located at 43.76°, -71.53° within the Merrimac River Watershed, is oligotrophic, and morphologically. Bennett Brook is a second order tributary to Squam Lake. Its 2.6 km 2 watershed comprises 2% of the 172 km 2 Squam Lake watershed.
In 2017, a high concentration of DDT residues (60 µg/kg) was reported in a sediment sample collected in 2016 at the mouth of Bennett Brook, on the northwest end of Squam Lake (Vogel, 2019). Within this watershed, a 0.25 km 2 orchard of 2300 apple trees surrounding Bennett Brook was actively managed from the early 1920s until 1961, at the latest. Commercial agriculture has not occurred on those lands since, so it has been almost 60 years since the last likely DDT application, making the maximum duration of applications 15 years, based on the rst availability of DDT in the U.S. around 1946. In the U.S., DDT use on orchards was common, especially to control codling moth (Glass and Fiori, 1955;Marshall, 1959). Today, remnants of the orchard operation include two dump sites and a barn. The barn was destroyed in a controlled burn around 1967 and, based on site observations today, the area was likely bulldozed afterwards, disrupting the barn's foundation.

Soil and sediment collection
Historic data on land uses and watershed characteristics guided strategic selection of sampling locations to determine DDT source area(s) (Fig. 1). Sur cial sediments were collected from the upper few centimeters of Bennet Brook's streambed deposits by scooping the material directly into new, clean, borosilicate glass jars. Outside the stream channel, a stainless-steel probe was used to collect the upper few centimeters of O and A soil horizons, dropped directly into sample jars. Samples were refrigerated until processing, then air dried for 48 hours and passed through a 250 µm sieve, retaining the ner fraction.
Samples were processed largely following U.S. EPA method 3546, using a microwave digestor (CEM MARS 5), 14 mL vessels, and 1.25 g of sample mixed with 10 mL acetone/hexane (50/50 v/v) extraction solvent in each vessel. During extraction, the vessels were heated to 120°C in 10 minutes, followed by a 20 minute hold time and 15 minute cooling time. Because the vessels were smaller than the desired sample size, each sample was split across three vessels for extraction and recombined for analysis. For each sample, the liquid extract was collected and combined by passing the contents of the three vessels through a glass column with a fritted disc. Afterwards the samples were dried through a column with sodium sulfate, concentrated in a Kuderna-Danish (K-D) concentrator, and cleaned with orisil (1 g, 6 mL).

Lake core collection
Lake deposits can preserve the chronological sequence of sediments (Last and Smol, 2001) and lake sediment cores, which collect a top to bottom column of sediment, can improve understanding of historical and current levels of DDT residues in lakes (Oliver et al., 1989;Muir et al., 1995;Olsson et al., 2000;Hu et al., 2010;Kurek et al., 2019). Analyses of Squam Lake sediment cores, therefore, provides insight for: 1) the duration of Bennett Brook sourcing DDT to the lake; 2) the historic and current levels the aquatic ecosystem has been and is exposed to; 3) the dominant DDT products in the lake, and; 4) triggers for mobilization from the Bennett Brook watershed to Squam Lake.
Ground penetrating radar (GPR), which creates images of subsurface deposits, can help identify locations with undisturbed, layered sediments. In February 2019, we collaborated with engineer Dr. Steven Arcone, Dartmouth, to identify suitable lake coring locations. Based on three, 130 m-long over-ice GPR transects, radiating lakeward from the mouth of Bennett Brook, we identi ed the edge of the littoral slope and, in March 2019, we collected three freeze cores, spaced 5 m apart, at 10-12 m water depth. Freeze cores preserve the very top, highly occulent, layers of sediments usually disturbed and sometimes lost by traditional corers. This location, proximal to the out ow of Bennett Brook, should have sediments predominantly sourced from the Bennett Brook sub-watershed (Fig. 1). After removal of non-in situ sediments, we photographed and subsampled the frozen core at 1-cm intervals, using a band saw, and cleaned the subsamples with razor blades prior to thawing. Samples from SQ2019-2 (41 cm length) provided dried, homogenized material for 210 Pb, 214 Pb and 137 Cs analyses on a germanium gamma detector at Woods Hole Oceanographic Institution. By subtracting 214 Pb from 210 Pb, we calculate unsupported (excess) 210 Pb, assumed to accumulate in the sediments at a constant rate from atmospheric sources. The relationship of decay versus accumulation rate provides an estimate of elapsed time. Samples from SQ2019-3 (56 cm length) provided material for DDT residue analysis.

Cray sh collection
Cray sh reside and feed in sediments and are an important part of an aquatic food chain that includes sh, loons, heron, mink, otter, and racoons as top predators. Cray sh accumulate DDT by direct ingestion of DDT-laden sediments or consuming prey with DDT in their fatty tissues (Schilderman et al., 1977). Prolonged contamination of cray sh arises as persistent DDT in soils mobilizes, through soil erosion and transport, into streams and lake sediments (Dimond et al., 1968). Since cray sh reside in an area-bound region, their contaminant load represents conditions at the collection site and makes them a useful pollution indicator species (Schilderman et al., 1977).
Northern cray sh (Faxonius virilis) collections, obtained using beef liver lures at three locations in 2019, come from 1) the mouth of Bennett Brook; 2) a channel connecting Squam Lake to Little Squam Lake; and 3) Mirror Lake in Woodstock, NH (Fig. 1). Mirror Lake is a regional reference site, chosen for its proximity to Squam Lake and watershed with relatively low development. This study also includes extraction and analyses of six cray sh collected by the Loon Preservation Committee (LPC) in 2013, approximately 2.3 km and 1.9 km distant from the mouth of Bennett Brook. Cray sh sample processing follows a methodological variation of Tsygankov and Boyarova (2015), with Soxhlet extraction, rotary evaporation, puri cation with concentrated sulfuric acid, and drying through a sodium sulfate column, another rotary evaporation, reconstitution, and cleaning with orisil (1 g, 6 mL). All samples are of single cray sh, except one composite sample of three small cray sh (Table 1).

Analytical method and quanti cation
Soil, sediment, and cray sh samples analyzed at Plymouth State University used a gas chromatographyelectron capture detector (Agilent 7820a GC-ECD) instrument, with helium as the carrier gas and nitrogen as the makeup gas. Samples injected into the capillary column DB-608 (30 m x 0.25 mm ID x 0.25 µm), and a subsequent DB-1701P column of the same dimensions, con rm presence of analytes and their respective concentrations. Two µL of sample was injected into one column at a time, with a 10:1 split for soil and sediment analysis and 5:1 for cray sh analysis. During the runs, temperatures of the detector and inlet were 280°C and 200°C, respectively. The oven was held at 150 ºC for 0.5 minutes and heated to 270ºC at a rate of 6 ºC/min, then held at 270ºC for 10 minutes.
Samples were quanti ed using an external standard calibration with a minimum of four standards per isomer: p,p' DDT, p,p' DDE, and p,p' DDD, collectively referred to as ΣDDT. ΣDDT is reported with molecular weight adjustment, expressed in terms of the parent compound, p,p' DDT (FAO, 2016). Our reporting limit is equivalent to the concentration of the lowest calibration standard which is 7 µg/kg for soil and sediment samples, and 2 µg/kg for cray sh samples. Detections below these values are reported as below the reporting limit (BRL). Samples with no measurable detections are reported as non-detects (ND).

Quality Control and Cleanup
To con rm the absence of analytes in the solvent, a solvent blank was analyzed at the start and end of each batch of samples. Each analytical batch contained a split sample, with known amounts of p,p' DDT, p,p' DDE, and p,p' DDD added to one half before extraction. Based on this, extraction e ciencies for p,p' DDT, p,p' DDE, and p,p' DDD were 42.4% -159.7%, 52.6% -140.0%, and 137.6% -189.9%, respectively. This variability is probably caused by the heterogenous nature of the soil and sediment samples. Extraction e ciencies for p,p' DDD were always greater than 100%, because of DDT degradation into DDD within the GC-ECD instrument. From the beginning of the analytical work, within-instrument degradation was observed, but only when DDT was present in the sample, such as with each spiked sample. Recommended solutions were followed to resolve these degradation issues, including addition of a cleanup step with orisil, a Restek Siltek guard column (5 m x 0.25 mm ID), and ultra-inert liners. The guard column was inserted into the inlet, and the other end into a connector piece that joins it with the analytical column. After each run, 6 inches of the guard column was trimmed at the inlet end, and if necessary, a new ultra-inert liner was installed. DDT standards were placed as quality control checks throughout the sample sequence to monitor DDT degradation; these con rmed the added cleanup steps greatly reduced degradation, but did not entirely eliminate it. One soil sample was split into two, with half going to Alpha Analytical, Westborough, MA, U.S.A., a commercial environmental testing laboratory, and half to our lab.

Bennett Brook watershed soils and sediments
The analyses reveal measurable amounts of p,p' DDT and p,p' DDE but not p,p' DDD, in soil and stream sediment samples from the Bennett Brook watershed (Fig. 2, Table 2). While no stream sediments have more p,p' DDT than p,p' DDE, 75% of soil samples with detections above the reporting limit have a preponderance of p,p' DDT. Of those samples, p,p' DDT:p,p' DDT + p,p' DDE, is from 0.57 to 0.74. We also nd that DDT residues are widely distributed within the relict apple orchard. For soil samples collected outside the apple orchard along NH RTE 113, 90% contain no detectable DDT residues. Areas downgradient of dump sites and at tributary con uences, showed low or no detections of ΣDDT.
Elevated p,p' DDE concentrations occur in a NH RTE 113 gulley receiving runoff from the northern orchard area (the upper orchard). Of special note, soil samples from material overlying a bulldozed barn foundation have low detections, but soil samples B42, B53, and B54 contain DDT residues up to 53x higher than any other sample collected in the watershed (Fig. 2). These high-value samples are from a location just 2 meters east and downgradient of the bulldozed barn foundation (the barn site). Half of sample B53 was analyzed at Alpha Analytical, reporting 58% p,p' DDE. This compares well to our lab's results of 57% p,p' DDT, on the other split of this sample.
The barn site values range from 165.1 to 723.2 µg/kg p,p' DDT and 57.8 to 721.1 µg/kg p,p' DDE, with averages of 336.5 ± 264.3 µg/kg and 270.1 ± 305.7 µg/kg, respectively. ΣDDT levels are 232.1 to 1527.0 µg/kg and average 637.5 ± 604.4 µg/kg. The barn site's samples are signi cantly different from all other samples in the Bennett Brook watershed, based on a Mann-Whitney U test (W = 406.00, p = 0.006). A point source is a highly concentrated area of contamination, at an explicit location. We conclude that the barn site meets the criteria for a point source.  (Fig. 3).

The DDT residue levels in Bennett
Brook and Squam Lake sediments are of comparable magnitude to those found in other locales (Table  3).

Cray sh
Cray sh results are shown in Table 3. In Mirror Lake, the reference site, cray sh contain no detectable DDT residues. Cray sh from Bennett Brook have p,p' DDE levels of 0.7 to 12.6 µg/kg wet weight (ww) but neither p,p' DDT or p,p' DDD were detected. The p,p' DDE values in cray sh collected in 2019 at Squam Lake, distant from Bennett Brook, range from no detection to 0.8 µg/kg ww. A 2013 cray sh study in Squam Lake found a p,p' DDE range of 0.29 -1.1 µg/kg ww (Loon Preservation Committee, unpublished data). The levels of p,p' DDE in Bennett Brook cray sh (mean = 5.03) are statistically different than the other Squam Lake cray sh we analyzed (mean = 0.83), based on a 2 sample t-test (df = 8, t = 2.42, p = 0.042). Residues in samples that were below the reporting limit were assigned the lowest standard value of 2 µg/kg in the statistical analysis. Regression analyses reveal no relationship between carapace length and p,p' DDE levels (df = 12, F = 3.82, p = 0.076).

Sources of DDT
DDT residues in Bennett Brook sediments likely derive from multiple sources. DDT can remain in soil for years after initial application (Gao et al., 2013), which explains the presence of DDT residues throughout the orchard area. The orchard lands are located updrainage and on both sides of Bennet Brook. This means that, as the soils erode and are transported within the drainage, all of the orchard's soils might be nonpoint sources to Bennett Brook and, eventually, to Squam Lake. Furthermore, DDT contamination is largely constrained to the historical orchard operation, with 90% of samples collected outside the orchard showing no detection of residues. Therefore, DDT residues detected in Bennett Brook are most likely sourced from legacy contamination from past DDT use on the orchard.
The high DDT levels detected at the barn site indicate probable storage there during the historical orchard operation, and failure to remove stored DDT prior to destruction of the barn. Of the barn site samples, B42 has the highest levels of residues (1,527.0 µg/kg ΣDDT). This sample included the deepest material collected (to 6 cm depth), suggesting residues may be concentrated in the A horizon, below the O horizon.
The vertical extent of the contamination is as yet unknown but could be determined by collecting successively deeper samples until detections diminish. However, due to low mobility of DDT and potentially low soil invertebrate activity, vertical movement may be limited (Boul et al., 1994;Dimond and Owen, 1996;Kaste et al., 2007). Our study included only six samples in and around the barn. More intensive sampling of the site is needed to assess the spatial distribution and range of levels at the location, and overall threat of this site as a point source.
Within the study area, roadside gullies along NH RTE 113 carry runoff, including any DDT-laden sediments eroded from upslope locations, directly into Bennett Brook. We nd p,p' DDE in the gully samples along the stretch of NH RTE 113 immediately downgradient of the orchard area, upgradient from the barn site. We nd no detection in roadside gullies in the study area that are not adjacent to the orchard lands or barn. This nding supports the idea that DDT is not just from a single point source at the barn but is also transported to Bennett Brook by runoff from DDT-treated soils. It also excludes the road itself as a source. The upper orchard is steeply sloped and was recently logged. Steep slopes have higher runoff and greater potential for erosion, even without the soil destabilization caused by logging (Tang et al., 2014). Soils eroded from the upper orchard may be nonpoint sources of residues adsorbed to the soils from historic applications, perhaps even more so because of their vulnerability to erosion (Munn and Gruber, 1997). Since samples were not collected from the soils within the upper orchard, we cannot rule out the possibility that additional point sources may contribute to p,p' DDE in the gullies.

Transportation potential
Page 12/22 The peak values for p,p' DDD and p,p' DDE in the lake core sediments occurs in those dated to 1982, ten years after the U.S. ban. This lag in peak concentrations implies retention of DDT-laden particles in the watershed soils or in Bennett Brook's channel deposits (Muir et al., 1995;Kurek et al., 2019). We hypothesized that watershed erosion events would produce spikes of ΣDDT in the lake sediments, associated with oods and other sediment disturbances. The freeze core was subsampled at 0.5 cm increments to enable this kind of event identi cation. However, none of the extreme oods and sediment transport events associated with beaver dam breaches in 2002, the Mother's Day Storm in 2006, or Tropical Storm Irene in 2011 leave a distinct DDT signature in the lake core collected in 2019, although we clearly see debris horizons in the cores from those events. Higher resolution core subsampling, for instance at 0.25 cm intervals, may provide more information on impacts from individual storms.
The lake core results show that DDT residues have consistently entered Squam Lake for several decades, from a seemingly constant source, or sources, in the Bennett Brook watershed. Weathered soils can release persistent contaminants for years after last application (Santschi et al., 2001;Hu et al., 2010;Bettinetti et al., 2016). Kaste et al. (2007) nd that New England soils are not easily eroded and, for soil mobilization to occur, the source soils must be especially vulnerable to erosion, such as stream banks, logged land, and steep slopes. Transportation of erodible sources can occur even at low rain intensities if the soils are unstable. The constant supply observed in the lake core is likely transported from multiple sources, including erodible orchard soils, the barn site, at culverts and behind beaver structures, and perhaps another unidenti ed point source in the upper orchard.
High levels in the barn site samples (max = 1,527.0 µg/kg ΣDDT), but not in samples downgradient of the barn (max = 34.8 µg/kg ΣDDT) may suggest stable DDT-laden soils at the barn site or a relatively slow transportation rate. However, as previously mentioned, more sampling around and downgradient of the barn is needed to better understand stability of the soils and transportation rates.

Persistence
Historically, when higher levels of DDT than DDE occur in soil, it serves as a possible indicator of illegal usage or dumping (Hitch and Day, 1992). However, as recent use cannot be completely ruled out, we have no reason to suspect illegal DDT usage in the study area. Supporting this, another study investigating the cause of unusually high DDT:DDE in soils concluded the cause was slow conversion of old DDT, applied before the ban, to DDE (Hitch and Day, 1992). Yang et al. (2013) and Sánchez-Osorio et al. (2017) nd p,p' DDT as the dominant isomer in soil samples, and the latter study concluded the lack of microbial degradation of p,p' DDT might account for its dominance in some soil samples analyzed in the study.
The presence of higher concentrations of p,p' DDT than p,p' DDE at the barn and other soil samples, but not in Bennett Brook sediments, suggests that p,p' DDT is preserved while in the soils but degrades to p,p' DDE once it enters Bennett Brook. This is consistent with studies that indicate longer persistence in soils than in mobilized sediments (Johnson et al., 1988;Pham et al., 1993).
Slower degradation is expected in the cold climates of New Hampshire, which has an average temperature of 6°C (Pham et al., 1993;Dimond and Owen, 1996;U.S. Geological Survey, 2016). Flooding, even of short duration, reduces the persistence of DDT by increasing anaerobic microbial activity that break down DDT into DDD by reductive dechlorination (Boul et al., 1994). The preponderance of DDE in the watershed's soils, therefore, is indicative of aerobic conditions. Also, the soils at the barn site are dominantly sandy (53% sand, 41% silt, and 6% clay). Longer DDT half-lives are likely in sandy soils, because they drain water rapidly and have low water-holding capacities (Crowe and Smith, 2007;National Cooperative Soil Survey, 2019). Lichtenstein (1971) shows that DDT is more persistent and stable in soils that are applied with high doses of the insecticide. Also, Pereira et al. (1996) reports that a point source with high concentrations of DDT may resist degradation to a greater degree than a site with lower concentrations, or from nonpoint or diffuse sources. Degradation occurs more rapidly at some distance away from a point source, yielding higher DDE:DDT values further from the source, and vice versa at the point source (Pereira et al., 1996).
This is because aerobic microbial activity is inhibited in the presence of high DDT levels. Also, because the insecticide eliminates soil-dwelling organisms, bioturbation and soil decomposition in these areas are minimized. Both effects result in longer persistence (Nash and Woolson, 1967;Pereira et al., 1996). At the barn site, soil microbial activity could be readily compared with locations out of the zone of contamination, providing more insight on this idea of persistence.
Although p,p' DDT was not detected in the lake sediments, this is not surprising given the rapid degradation rate of DDT to DDD by reductive dechlorination under anaerobic conditions (Wedemeyer, 1966;Miles and Harris, 1973;Johnson et al., 1988;Pham et al., 1993;Pereira et al., 1996). Supporting this, we nd DDD:DDE in the lake sediments are always greater than 1, indicating dominance of anaerobic conditions in the lake depositional environment (Zhang and Shan, 2014;Ma et al., 2016). Other studies also report higher DDD than DDT and DDE in lake sediments, since anaerobic conditions are common in lake bottoms (Oliver et al., 1989;Muir et al., 1995;Kurek et al., 2019).

Aquatic organisms
Since cray sh only travel up to a few hundred meters, cray sh collected in Bennett Brook represent those that live in or just outside the brook (Byron and Wilson, 2001;Craddock 2009). Therefore, p,p' DDE detected in Bennett Brook-residing cray sh likely derives from sources in the Bennett Brook watershed. Cray sh collected in Bennett Brook have signi cantly higher concentrations of p,p' DDE than cray sh collected elsewhere in Squam Lake, distant from the brook. Therefore, cray sh results show that DDT residues sourcing from the Bennett Brook watershed have entered the aquatic food chain, at levels signi cantly higher than distant from the brook.
We hypothesized that DDE levels would increase with cray sh carapace length because larger, older cray sh accumulate residues over a longer interval than smaller, younger cray sh. Supporting this hypothesis, in sh, the accumulation of DDE is correlated with increasing age and fat content (Gutenmann et al., 1992;USDOI, 1998). Statistical analyses revealed no relationship between cray sh size and DDE levels in the cray sh we analyzed. However, the range of sizes we tested may not have represented enough a range to see this effect.
Cray sh usually have higher DDE than DDT, because the aerobic DDT-laden sediments they reside in have degraded to DDE, and DDT also breaks down in their bodies (Dimond et al., 1968;Boul, 1995). Prolonged cray sh contamination occurs through persistence of DDT residues in contributing soils, and residues in cray sh will accumulate as long as the watershed soils provide that input (Dimond et al., 1968). Results reveal that the Bennett Brook watershed continues to supply DDT-laden soils.
Most sediment samples analyzed from Bennett Brook and Squam Lake exceed sediment quality guidelines for the protection of aquatic life (CCME, 2001). For example, p,p' DDD and p,p' DDE concentrations in the upper 5 cm of the lake sediments are ve and six times higher, respectively, than their PELs (Probable Effects Level), above which adverse biological are expected to occur frequently (CCME 2001). Exceeding the PELs of DDT, DDE, and DDD at 4.77, 6.75, and 8.51 μg/kg, respectively, means that the levels are potentially harmful to Squam Lake's ecosystem.

Conclusions
DDT usage in the U.S. spans about 27 years, but it was only used on the Bennett Brook area orchards for a maximum of 15 years. Although it has been almost 60 years since the last application, elevated ΣDDT is still detected in the Bennett Brook watershed soils, sediments, and cray sh and in Squam Lake sediments. In this study, we demonstrate that DDT residues are transported from watershed soils to stream and lake sediments and into the aquatic food chain. This watershed systems approach aids in understanding the extent of the contamination.
The orchard is a source of DDT to Bennett Brook and Squam Lake. Past and present sources include erodible orchard soils serving as a nonpoint source, the barn site, at culverts, behind beaver structures, and perhaps other unidenti ed point sources. A preponderance of p,p' DDT in most soil samples but not in sediment samples, suggests degradation once transported into Bennett Brook and Squam Lake. Based on lake core results, Squam Lake's food chain has been exposed to DDT residues at potentially harmful levels to the ecosystem since 1951. ΣDDT levels in the lake sediments have a decreasing trend from 1982 to the present. Cray sh results show that DDT residues sourcing from the Bennett Brook watershed have entered the aquatic food chain, at levels signi cantly higher than distant from the brook. Further research should be done to determine the constraints and mobilization potential of DDT contamination at the barn site, and to understand if Squam Lake's aquatic life is experiencing adverse effects from the contaminant. Study area in central New Hampshire, U.S. with sampling locations for cray sh, soils, stream sediments, and lake cores. p,p' DDE, p,p' DDD, and ΣDDT concentrations from the lake core SQ2019-3. The vertical bars represent the 210Pb error