USGS Groundwater Dating Lab

20 May USGS Groundwater Dating Lab

The slowing and turnover of CFC atmospheric mixing ratios in the 1990s has resulted in ambiguity in CFC-based apparent age. Sulphur hexafluoride has been developed as an alternative to CFCs for gas-solubility dating of water . Significant https://lovematchcritic.com/oasisactive-review/ production of SF6 began in the 1960s for use in high voltage electrical switches. SF6 is extremely stable, with an estimated atmospheric lifetime of 800 (Morris et al., 1995) to 3200 years (Ravishankara et al., 1993).

Samples 7 and 8 are from areas in southern Idaho with deep unsaturated zones. Sample 7 is from a monitoring well in the Snake River Plain aquifer, southern Idaho, in an area where groundwater is being recharged by seepage from a nearby stream. The unsaturated zone consists of 16.8 m of alluvial sand that overlies 50.3 m of fractured-basalt and sedimentary-interbed sequence above the water table.

However, shallow ground-water supplies are generally young and, because there has been a wide variety of man-made pollutants produced in the 20th century, are more susceptible to contamination than deeper ground water. Solute transport in karst aquifers is primarily constrained to relatively complex and inaccessible solution conduits where transport is often rapid, turbulent, and at times constrictive. Breakthrough curves generated from tracer tests in solution conduits are typically positively-skewed with long tails evident. Physical nonequilibrium models to fit breakthrough curves for tracer tests in solution conduits are now routinely employed. Chemical nonequilibrium processes are likely important interactions, however.

Int. J. Appl. Radiat. Isotopes

The purpose of this article is to consider the applicability of backward-in-time techniques as tools for simulating reactive transport in porous media. A multi-component reaction system is considered in a variety of systems of increasing complexity and we show that complex, non-linear systems can be simulated backward in time, given a sufficiently robust integration scheme. Recent advances in reactive random walk particle tracking are employed to investigate simple flow systems with spatially variable reactions, as well as 2-d heterogeneous flows, and we show that some level of time reversibility exists in both cases. Under a uniform injection scheme, the total masses generated in forward and backward simulations of the 2-d models were all within 3.5% of each other for all the species considered, indicating good overall agreement between the models.

Time series chloride and δ18O and δ2H rainfall and production bore data suggest that there was a freshening (i.e. lower salinity input) of the lens as a result of recharge during the wet season, which occurs up to 6 months after the event. Groundwater residence time indicators showed that the mean residence time in the lens was at least 25 years and the rate of recharge to the system was up to 200 mm y−1. The study site is near the town of Strathroy in southwestern Ontario within a flat-lying agricultural area referred to as the Cardoc sand plain .

Sampling

Two thermokarst landscapes, the Chuluut (W-05, W-06) and the Galuut (W-14, W-14′), exist on the northern and southern sides, respectively. Spring discharge at Udleg (W-27), which is in the vicinity of Ulaanbaatar city, was also sampled for comparison. Table 1 shows the geographical coordinates and altitudes of the spring discharge sites. Where R is the effective recharge (md−1), L is the distance between hydrological boundaries , m is a coefficient equal to 8 for rectangular areas or 16 for circular shapes, k is the hydraulic conductivity (md−1), H is the saturated thickness and d is the maximum terrain raise . For a topography controlled groundwater table, local circulation dominates total flow, whereas intermediate and regional circulation is predominant in aquifers with a recharge controlled groundwater table . Spatially, the characteristics of groundwater parameters at the valley side were different from that in the middle part of the wetland.

Previous estimates based on carbon-14 measurements of unsaturated-zone carbon dioxide indicated a residence time of less than 40 years. In addition to decreasing the estimated travel time by an order of magnitude, the CFC data allowed gas circulation patterns in the mountain to be identified and quantified. These results are highly relevant to issues pertaining to isolation of possible repository-generated radioactive 14CO2.

Recently, the subject of transient age has become more prevalent in groundwater , hillslope (Fiori and Russo, 2008, Duffy, 2010, Fiori, 2012, Ali et al., 2014) and catchment (Botter et al., 2011, Benettin et al., 2013) studies. Many of these studies used relaxation methods or transfer functions but this ignores the spatial redistribution of the water mass, which is itself a transient process. However, even these modeling techniques suggest that the role of transience can be significant. Spring discharges in the region originate from permafrost groundwater that is mixed with ground ice-melt water and seasonally recharged by modern precipitation . Thus, the monitoring of spring discharge and the contribution ratio of both waters provides meaningful information for vulnerability assessment of water resources in the region.

Industrial production of SF6 began in 1953 with the introduction of gas-filled high-voltage electrical switches. The historical atmospheric mixing ratio of SF6 is being reconstructed from production records, archived air samples, and atmospheric measurements, and retrieved from concentrations measured in seawater and in previously-dated ground water. A preliminary reconstruction has been made of northern-hemisphere SF6 mixing ratios .

In this study, groundwater dating (3H–3He and time series of 3H) was combined with chemical and stable isotope analyses to resolve the evolution of nitrate sources and the driving mechanism of nitrate contamination. Approximately 75% of the groundwater samples had nitrate concentrations exceeding World Health Organization’s guideline for drinking water (50 mg/L), and 44% exceeded the groundwater quality standard of China (88.6 mg/L), indicating severe nitrate pollution. The shift of nitrate sources in different land use was identified using stable isotope composition of nitrate and groundwater age.