James B. Shanley

Who needs environmental monitoring

Environmental monitoring is often criticized as being unscientific, too expensive, and wasteful. While some monitoring studies do suffer from these problems, there are also many highly successful long-term monitoring programs that have provided important scientific advances and crucial information for environmental policy. Here, we discuss the characteristics of effective monitoring programs, and contend that monitoring should be considered a fundamental component of environmental science and policy. We urge scientists who develop monitoring programs to plan in advance to ensure high data quality, accessibility, and cost-effectiveness, and we urge government agencies and other funding institutions to make greater commitments to increasing the amount and long-term stability of funding for environmental monitoring programs.

Dissolved organic nitrogen budgets for upland, forested ecosystems in New England

Relatively high deposition ofnitrogen (N) in the northeastern United States hascaused concern because sites could become N saturated.In the past, mass-balance studies have been used tomonitor the N status of sites and to investigate theimpact of increased N deposition. Typically, theseefforts have focused on dissolved inorganic forms ofN (DIN = NH4-N + NO3-N) and have largelyignored dissolved organic nitrogen (DON) due todifficulties in its analysis. Recent advances in themeasurement of total dissolved nitrogen (TDN) havefacilitated measurement of DON as the residual of TDN− DIN. We calculated DON and DIN budgets using data onprecipitation and streamwater chemistry collected from9 forested watersheds at 4 sites in New England. TDNin precipitation was composed primarily of DIN. Netretention of TDN ranged from 62 to 89% (4.7 to 10 kghaminus 1 yrminus 1) of annual inputs. DON made up themajority of TDN in stream exports, suggesting thatinclusion of DON is critical to assessing N dynamicseven in areas with large anthropogenic inputs of DIN.Despite the dominance of DON in streamwater,precipitation inputs of DON were approximately equalto outputs. DON concentrations in streamwater did notappear significantly influenced by seasonal biologicalcontrols, but did increase with discharge on somewatersheds. Streamwater NO3-N was the onlyfraction of N that exhibited a seasonal pattern, withconcentrations increasing during the winter months andpeaking during snowmelt runoff. Concentrations ofNO3-N varied considerably among watersheds andare related to DOC:DON ratios in streamwater. AnnualDIN exports were negatively correlated withstreamwater DOC:DON ratios, indicating that theseratios might be a useful index of N status of uplandforests.

Sources, transformations, and hydrological processes that control stream nitrate and dissolved organic matter concentrations during snowmelt in an upland forest

We explored catchment processes that control stream nutrient concentrations at an upland forest in northeastern Vermont, USA, where inputs of nitrogen via atmospheric deposition are among the highest in the nation and affect ecosystem functioning. We traced sources of water, nitrate, and dissolved organic matter (DOM) using stream water samples collected at high frequency during spring snowmelt. Hydrochemistry, isotopic tracers, and end-member mixing analyses suggested the timing, sources, and source areas from which water and nutrients entered the stream. Although stream-dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) both originated from leaching of soluble organic matter, flushing responses between these two DOM components varied because of dynamic shifts of hydrological flow paths and sources that supply the highest concentrations of DOC and DON.

The effect of frozen soil on snowmelt runoff at Sleepers River, Vermont†

Soil frost depth has been monitored at the Sleepers River Research Watershed in northeastern Vermont since 1984. Soil frost develops every winter, particularly in open fields, but its depth varies greatly from year to year in inverse relation to snow depth. During the 15 years of record at a benchmark mid-elevation open site, the annual maximum frost depth varied from 70 to 390 mm. We empirically tested the hypothesis that frozen soil prevents infiltration and recharge, thereby causing an increased runoff ratio (streamflow/(rain + snowmelt)) during the snowmelt hydrograph rise and a decreased runoff ratio during snowmelt recession. The hypothesis was not supported at the 111 km 2 W-5 catchment; there was no significant correlation of the runoff ratio with the seasonal maximum frost depth for either the pre-peak or post-peak period. In an analysis of four events, however, the presence of frost promoted a large and somewhat quicker response to rainfall relative to the no-frost condition, although snow cover caused a much greater time-to-peak regardless of frost status. For six years of flow and frost depth measured at the 59 ha agricultural basin W-2, the hypothesis appeared to be supported. The enhancement of runoff due to soil frost is evident on small plots and in extreme events, such as rain on frozen snow-free soil. In the northeastern USA rind eastern Canada, the effect is often masked in larger catchments by several confounding factors, including storage of meltwater in the snowpack, variability in snowmelt timing due to elevational and aspect differences, interspersed forested land where frost may be absent, and the timing of soil thawing relative to the runoff peak.

Factors Controlling Mercury Transport in an Upland Forested Catchment

Total mercury (Hg) deposition and input/output relationships were investigated in an 11-ha deciduous forested catchment in northern Vermont as part of ongoing evaluations of Hg cycling and transport in the Lake Champlain basin. Atmospheric Hg deposition (precipitation + modeled vapor phase downward flux) was 425 mg ha-1 during the one-year period March 1994 through February 1995 and 463 mg ha-1 from March 1995 through February 1996. In the same periods, stream export of total Hg was 32 mg ha-1, respectively. Thus, there was a net retention of Hg by the catchment of 92% the first year and 95% the second year. In the first year, 16.9 mg ha-1 or about half of the annual stream export, occurred on the single day of peak spring snowmelt in April. In contrast, the maximum daily export in the second year, when peak stream flow was somewhat lower, was 3.5 mg ha-1 during a January thaw. The fate of the Hg retained by this forested catchment is not known. Dissolved (< 0.22 µm) Hg concentrations in stream water ranged from 0.5-2.6 ng L-1, even when total (unfiltered) concentrations were greater than 10 ng L-1 during high flow events. Total Hg concentrations in stream water were correlated with the total organic fraction of suspended sediment, suggesting the importance of organic material in Hg transport within the catchment. High flow events and transport with organic material may be especially important mechanisms for the movement of Hg through forested ecosystems.

Comparison of total mercury and methylmercury cycling at five sites using the small watershed approach

The small watershed approach is well-suited but underutilized in mercury research. We applied the small watershed approach to investigate total mercury (THg) and methylmercury (MeHg) dynamics in streamwater at the five diverse forested headwater catchments of the US Geological Survey Water, Energy, and Biogeochemical Budgets (WEBB) program. At all sites, baseflow THg was generally less than 1ng L(-1) and MeHg was less than 0.2ng L(-1). THg and MeHg concentrations increased with streamflow, so export was primarily episodic. At three sites, THg and MeHg concentration and export were dominated by the particulate fraction in association with POC at high flows, with maximum THg (MeHg) concentrations of 94 (2.56)ng L(-1) at Sleepers River, Vermont; 112 (0.75)ng L(-1) at Rio Icacos, Puerto Rico; and 55 (0.80)ng L(-1) at Panola Mt., Georgia. Filtered (<0.7microm) THg increased more modestly with flow in association with the hydrophobic acid fraction (HPOA) of DOC, with maximum filtered THg concentrations near 5ng L(-1) at both Sleepers and Icacos. At Andrews Creek, Colorado, THg export was also episodic but was dominated by filtered THg, as POC concentrations were low. MeHg typically tracked THg so that each site had a fairly constant MeHg/THg ratio, which ranged from near zero at Andrews to 15% at the low-relief, groundwater-dominated Allequash Creek, Wisconsin. Allequash was the only site with filtered MeHg consistently above detection, and the filtered fraction dominated both THg and MeHg. Relative to inputs in wet deposition, watershed retention of THg (minus any subsequent volatilization) was 96.6% at Allequash, 60% at Sleepers, and 83% at Andrews. Icacos had a net export of THg, possibly due to historic gold mining or frequent disturbance from landslides. Quantification and interpretation of Hg dynamics was facilitated by the small watershed approach with emphasis on event sampling.

Ultraviolet absorbance as a proxy for total dissolved mercury in streams

Stream water samples were collected over a range of hydrologic and seasonal conditions at three forested watersheds in the northeastern USA. Samples were analyzed for dissolved total mercury (THg(d)), DOC concentration and DOC composition, and UV(254) absorbance across the three sites over different seasons and flow conditions. Pooling data from all sites, we found a strong positive correlation of THg(d) to DOC (r(2)=0.87), but progressively stronger correlations of THg(d) with the hydrophobic acid fraction (HPOA) of DOC (r(2)=0.91) and with UV(254) absorbance (r(2)=0.92). The strength of the UV(254) absorbance-THg(d) relationship suggests that optical properties associated with dissolved organic matter may be excellent proxies for THg(d) concentration in these streams. Ease of sample collection and analysis, the potential application of in-situ optical sensors, and the possibility for intensive monitoring over the hydrograph make this an effective, inexpensive approach to estimate THg(d) flux in drainage waters.

Mercury on the move during snowmelt in Vermont

Although mercury (Hg) emissions peaked in the United States over the last 20 to 40 years and are now declining, they remain well above natural background levels in soils and sediments. Only a small fraction of the Hg deposited from the atmosphere to the terrestrial landscape runs off in streamflow. However, some of this Hg is methylated in the environment and can potentially bioaccumulate to the top of food webs, posing a hazard to people who eat fish, especially children and pregnant women. What factors determine the amount of Hg that runs off in streams? During the 2000 snowmelt at Sleepers River in Vermont, strong correlations were found between dissolved and particulate mercury and the respective dissolved and particulate organic carbon fractions, even when data were pooled from 10 streams of diverse watershed size and land cover. Episodic export of particulate Hg during the highest flows appears to be the dominant mechanism of Hg movement.

Comparison of atmospheric mercury speciation and deposition at nine sites across central and eastern North America

[1] This study presents >5 cumulative years of tropospheric mercury (Hg) speciation measurements, over the period of 2003-2009, for eight sites in the central and eastern United States and one site in coastal Puerto Rico. The purpose of this research was to identify local and regional processes that impact Hg speciation and deposition (wet + dry) across a large swath of North America. Sites sampled were selected to represent both a wide range of mercury exposure and environmental conditions. Seasonal mean concentrations of elemental Hg (1.27 ± 0.31 to 2.94 ± 1.57 ng m -3 ; x ± σ), reactive gaseous mercury (RGM; 1.5 ± 1.6 to 63.3 ± 529 pg m -3 ), and fine particulate Hg (1.2 ± 1.4 to 37.9 ± 492 pg m -3 ) were greatest at sites impacted by Hg point sources. Diel bin plots of Hg° and RGM suggest control by a variety of local/regional processes including impacts from Hg point sources and boundary layer/free tropospheric interactions as well as from larger-scale processes affecting Hg speciation (i.e., input of the global Hg pool, RGM formed from oxidation of Hg° by photochemical compounds at coastal sites, and elemental Hg depletion during periods of dew formation). Comparison of wet Hg deposition (measured), RGM and fine particulate Hg dry deposition (calculated using a multiple resistance model), and anthropogenic point source emissions varied significantly between sites. Significant correlation between emission sources and dry deposition was observed but was highly dependant upon inclusion of data from two sites with exceptionally high deposition. Findings from this study highlight the importance of environmental setting on atmospheric Hg cycling and deposition rates.

Preliminary observations of streamflow generation during storms in a forested Piedmont watershed using temperature as a tracer

Abstract Variations in streamwater temperature at the outlet of a 41-ha forested watershed at Panola Mountain in the Georgia Piedmont indicate that the initial rapid hydrologic response is caused by a combination of groundwater discharge and channel interception of rainwater. A storm in May 1986 caused a rapid increase in discharge that was accompanied by a decrease in streamwater temperature and a rise in the water table level adjacent to the stream. The higher water table provided the hydraulic gradient necessary to increase the discharge of colder groundwater to the stream. Storms that occurred under very dry antecedent conditions in July 1986 and June 1987 caused a rapid hydrologic response but no change in water table level, indicating the response was caused by channel interception of rainwater. This conclusion was supported by increases in streamwater temperature in the June storm and by chemical changes in the July storm. When rainfall is sufficient, flow in the ephemeral part of the stream in the catchment headwaters generates a second and larger discharge peak that reflects the chemistry and temperature of runoff from a 3-ha granite outcrop in the headwaters; sulfate concentration and temperature increase and alkalinity decreases relative to prestorm conditions. The initial response, however, results from channel interception and groundwater discharge. Rapid rises in the water table level during some storms suggest that macropore flow may play a major role in the hydrologic response of the watershed to rainstorms.