Sources of DDT
DDT residues in Bennett Brook sediments are likely from multiple sources. DDT applied to an area can remain in soil for years, which explains the presence of DDT residues throughout the orchard area. Thus, it is probable that the orchard, as a whole, serves as a nonpoint source of DDT to Bennett Brook and, eventually, to Squam Lake. Furthermore, DDT contamination is largely constrained to the orchard operation, since 90% of samples collected outside the orchard resulted in no detection of residues. Therefore, DDT residues detected in Bennett Brook are likely sourced from legacy contamination from past DDT use on the orchard (Gao et al. 2013).
Based on the high levels detected at the barn site, DDT may have been stored there during the orchard operation, resulting in a point source of the contaminant. Of the barn site samples, B42 has the highest levels of residues (1,527.0 µg/kg ΣDDT) and was the deepest sample collected in the soil horizon (0 – 6 cm), 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.
Roadside gullies along NH RTE 113 in the study area carry runoff, including any DDT-laden sediments, directly into Bennett Brook. We find p,p’ DDE in the gully samples along the stretch of NH RTE 113 that is immediately downgradient of the upper orchard area and upgradient from the barn site. This finding 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. 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 the soils 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 of additional point sources contributing to p,p’ DDE in the gullies.
Transportation potential
The main peak in p,p’ DDD and p,p’ DDE concentrations in the lake core sediments in 1982, ten years after the U.S. ban, indicates retention of DDT-laden particles in the watershed soils or Bennett Brook, creating a lag in delivery of residues to Squam Lake (Muir et al. 1995, Kurek et al. 2019).
We hypothesized that watershed erosion events would produce spikes of ΣDDT in the lake sediments, as floods and other disturbances mobilize stream sediments into Squam Lake. However, the extreme events of the Mother’s Day Storm in 2006 or Hurricane Irene in 2011, and the beaver ponds breach in the lower reaches of Bennett Brook in 2002, all fail to leave a distinct DDT signature in the lake core. Higher resolution subsampling, for instance at 0.25 cm intervals, may provide more information on impacts from individual storms. Nonetheless, since unstable soils are eroded and mobilized during events such as these, it is reasonable to assume that extreme events have and will continue to trigger mobilization of the contaminant. The destruction of the barn in a controlled burn in about 1967 destabilized soils on the barn floor and also, potentially, stored DDT, allowing them to mobilize from the barn and into Bennett Brook. Bulldozing around the barn’s foundation, which occurred sometime after the building was burned, likely increased the mobilization and flux of DDT.
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) find 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 unidentified 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) find 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 (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 find 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 crayfish only travel up to a few hundred meters, crayfish 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 crayfish likely derives from sources in the Bennett Brook watershed. Crayfish collected in Bennett Brook have significantly higher concentrations of p,p’ DDE than crayfish collected elsewhere in Squam Lake, distant from the brook. Therefore, crayfish results show that DDT residues sourcing from the Bennett Brook watershed have entered the aquatic food chain, at levels significantly higher than distant from the brook. However, fish and piscivorous birds, such as loons, kingfishers, and bald eagles, are not confined to as small a range as crayfish. They can easily travel and ingest prey with differing contaminant levels, depending on their location in the lake. Any such predator, whose range includes Bennett Brook, could experience higher DDT bioaccumulation.
We hypothesized that DDE levels would increase with crayfish carapace length because larger, older crayfish accumulate residues over a longer interval than smaller, younger crayfish. Supporting this hypothesis, in fish, the accumulation of DDE is correlated with increasing age and fat content (Gutenmann et al. 1992, USDOI 1998). Statistical analyses revealed no relationship between crayfish size and DDE levels in the crayfish we analyzed. However, the range of sizes we tested may not have represented enough a range to see this effect.
Crayfish 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 crayfish contamination occurs through persistence of DDT residues in contributing soils, and residues in crayfish 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, and so we expect contamination in the crayfish and Squam Lake food chain for many more years. Even low levels in crayfish should not be overlooked, because of biomagnification (Dimond et al. 1968).
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 five 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.