Estimated Mean Residence Time Research Articles

Hydrogeochemsitry of montane springs and their influence on streams in the Cairngorm mountains, Scotland

Abstract. Springs are important groundwater discharge points on the high altitude (>800m) plateaux of the Cairngorm mountains, Scotland and form important wetland habitats within what is often a dry, sub-arctic landscape. The hydrogeochemistry of a typical spring in the Allt a'Mharcaidh catchment was examined between 1995-98 in order to characterise its chemical composition, identify the dominant controls on its chemical evolution and estimate groundwater residence time using 18O isotopes. Spring water, sustained by groundwater flow in shallow drift deposits and fractured bedrock, was moderately acidic (mean pH 5.89), with a very low alkalinity (mean 18 μeq l-1) and the ionic composition was dominated by sea-salts derived from atmospheric sources. Geochemical modelling using NETPATH, predicted that the dissolution of plagioclase mainly controls the release of Si, non-marine Na, Ca, K and Al into spring water. Hydrological conditions influenced seasonal variations in spring chemistry, with snowmelt associated with more rapid groundwater flows and lower weathering rates than summer discharges. Downstream of the spring, the chemistry of surface water was fundamentally different as a result of drainage from larger catchment areas, with increased soil and drift cover, and higher evaporation rates. Thus, the hydrogeochemical influence of springs on surface waters appears to be localized. Mean δ18O values in spring water were lower and more damped than those in precipitation. Nevertheless, a sinusoidal seasonal pattern was observed and used to estimate mean residence times of groundwater of around 2 years. Thus, in the high altitude plateau of the Cairngorms, shallow, coarse drift deposits from significant aquifers. At lower altitudes, deeper drift deposits, combined with larger catchment areas, increase mean groundwater residence times to >5 years. At high altitudes, the shallow, permeable nature of the drifts dictates that groundwater is vulnerable to impacts of environmental changes that could be usefully monitored at spring sites.

The role of suspended particles in the discharge of 210Pb and 7Be within the Kuji River watershed, Japan

Fluvial discharge of atmospherically-derived 210Pb and 7Be was studied in the Kuji River watershed in Japan. The daily load of 210Pb and 7Be, associated with suspended particles, greatly increased during high-flow conditions. However, the activity concentrations of 210Pb and 7Be in the suspended particles were found to be rather independent of the flow rate. Due to this feature, the daily load of the activities associated with the suspended particles was mainly controlled by that of the suspended particles. Compared with the other reported values, the estimated mean residence time of atmospherically-derived 210Pb in the present study was rather short (about 800 years), possibly reflecting the high availability of the suspended particles eroded from the top soil in the watershed. The annual load of 210Pb and 7Be was predominantly of particulate form (approx 90%). These results suggest that the fluvial discharge of atmospherically-derived, highly adsorptive (k d ≳ 10 4 mg l −1) radionuclides in a river watershed is primarily due to the discharge of suspended particles, which strongly depends on the flow rate of river water and also on the location of the watershed.

Amikacin Pharmacokinetics in Patients with Spinal Cord Injury

The influence of chronic (greater than 1 yr duration) spinal cord injury (SCI) on the disposition of amikacin was studied in seven healthy subjects with SCI (five paraplegic, two tetraplegic) and seven able-bodied controls (intact neuraxes). The time course of amikacin serum concentration after a 30-minute infusion (7.5 mg/kg) was followed for up to 8.5 hours using fluorescence polarization immunoassay. Pharmacokinetic values were estimated by a noncompartmental analysis (NC). Amikacin steady-state volume of distribution (Vss) was increased to 0.20 +/- 0.04 l/kg (mean +/- SD) as compared to 0.17 +/- 0.02 l/kg in able-bodied controls (p 0.03), and its mean terminal elimination half-life in patients with SCI was prolonged by 0.64 hours over the control value of 2.11 +/- 0.27 hours (p 0.01). The NC estimated mean residence time (MRT) in patients with SCI (3.65 +/- 0.75 hrs) was 0.89 hours longer than that observed in controls (p 0.03). Our data suggest that the Vss, half-life, and MRT of amikacin are increased in persons with chronic SCI. As a result, amikacin dosing regimens developed in able-bodied humans may demonstrate diminished efficacy when extrapolated uncritically to these patients.

Plutonium and americium in arctic waters, the North Sea and Scottish and Irish coastal zones

Plutonium and americium have been measured in surface waters of the Greenland and Barents Seas and in the northern North Sea from 1980 through 1984. Measurements in water and biota, Fucus, Mytilus and Patella, were carried out in North-English and Scottish waters in 1982 and Fucus samples were collected from the Irish coast in 1983. Fallout is found to dominate as a source of 239+240Pu north of latitude 65°N, while for 238Pu a substantial fraction originates from European nuclear fuel reprocessing facilities. The 238Pu/ 239+240Pu isotope ratio provides clear evidence of the transport of effluent plutonium from the latter to Spitsbergen waters. Fallout plutonium in Arctic waters has a residence time of the order of several years, while for Pu from Sellafield we estimate mean residence times of 11–15 months in Scottish waters and, tentatively, 1·5-3 y during transport from the North Channel (north of the Irish Sea) to Spitsbergen. 241Am found in Arctic waters probably originates from the decay of fallout 241Pu and, like Pu, tentatively has a residence time of the order of several years. Americium from Sellafield has an estimated mean residence time of 4–6 months in Scottish waters.

Biogeochemistry of Lead in McDonalds Branch Watershed, New Jersey Pine Barrens

AbstractLead concentrations, fluxes, and storage were measured in the vegetation, forest floor, and acid, sandy mineral soil (Quartzipsamments) of a forested watershed in the New Jersey Pine Barrens. Atmospherically deposited Pb at the McDonalds Branch Watershed was 140 g ha−1 yr−1 (8 µg L−1 in bulk precipitation) in 1980 to 1982, a substantial reduction from 350 g ha−1 yr−1 (17 µg L−1 in bulk precipitation) in 1978 to 1979. Virtually all Pb falling on the uplands was retained—75% by the organic forest floor and 25% by mineral soils, especially in B and C soil horizons. Total Pb content of the forest floor was 7.6 kg ha−1, with an accumulation rate of 100 g ha−1 yr−1 (1980–1982), or 1.3% of the forest floor pool. Estimated mean residence time of Pb in the forest floor was 220 yr. About 35 g Pb ha−1 yr−1 moved out of the forest floor in solution through the E horizon and was correlated strongly with dissolved organic matter. Approximately 30 g Pb ha−1 yr−1 accumulated in the B and C soil horizons. Less than 1 g Pb ha−1 yr−1 percolated past a depth of 2 m. Lowland muck soils and vegetation accumulated 98% of incoming Pb, with only 3.2 g Pb ha−1 yr−1 exported in the stream. Lead concentrations in the stream were correlated positively with dissolved organic matter and water level in the swamps. Lead in biota was contained mainly in the bark, fine roots, and foliage. Concentrations were: fine roots (18 mg kg−1) > bark (15 mg kg−1) > foliage (4 mg kg−1) > wood (0.5 mg kg−1). Although Pb concentrations in herbs, mosses, and lichens ranged from 10 to 60 mg kg−1, these plants accounted for very little biomass. Total Pb content of biota was 335 g ha−1, about 4% of the forest floor Pb content.