Advances in Tensiometry for Long-term Monitoring of Soil Water Pressures

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In desert environments, nonvegetated (bare) soils and sediments can act as recharge basins, allowing water infiltration but restricting evaporation. When such sediments are located over buried wastes, drainage can transport vadose zone contamination downward to groundwater. Lysimeters were used to quantify drainage from bare sediments at the U.S. Department of Energy's Hanford Site in Washington state, USA. Drainage varied widely from zero to more than half of the annual precipitation for sediments ranging from fine silts to coarse rock fragments. Decade‐long drainage records were used to develop two empirical models relating annual drainage and textural properties of bare sediments. A 22‐yr drainage record for bare, coarse sand was tested, and the calibration developed for the past 10 years (1995–2004) was found to reliably predict drainage from the previous 12 years. The texture models were also compared against Darcy's Law drainage estimates (i.e., unsaturated hydraulic conductivity) for coarse sand and found to outperform Darcy's Law estimates of the long‐term drainage average. The texture models reasonably predicted annual drainage rates for bare sediments containing significant fines (materials less than 50 μm), but significantly overpredicted drainage rates for clean rock and gravels with little or no fines. The failure of the textural models with coarse materials containing minimal fines was attributed to advective drying. Drainage predictions using the textural models indicate that to minimize drainage only modest quantities of fines need to be added to the coarse sediments to substantially reduce the potential for groundwater contamination.

Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Measurement and Prediction of Deep Drainage from Bare Sediments at a Semiarid Site

Gee

Keller

Ward

2005

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“…The simulation domain was discretized into 750 nodes with node spacing of 0.01 m. The initial Cl − concentration in the lysimeter in 1978 was taken to be 88 mg L −1 based on Cl − analysis of archived soil from the lysimeter, and the Cl − input was assumed to be steady at 0.684 mg L −1 , on the basis of the assumptions of 190 mm of annual precipitation, 0.225 mg L −1 input, and a drainage rate of 62.5 mm yr −1 After drainage started in 1981, the water storage was nearly constant, there was no runoff, and the average evaporation rate was 127.5 mm yr −1 The simulations were run in an isothermal mode. The lower boundary condition was assumed to be unit gradient, based on the observed unit gradient conditions measured with tensiometers (Gee, 1987; Sisson et al, 2002). We ran the simulations two ways.…”

Section: Resultsmentioning

confidence: 99%

Chloride Mass Balance: Cautions in Predicting Increased Recharge Rates

Gee¹,

Zhang²,

Tyler³

et al. 2005

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phy et al., 1996;Tyler et al., 1996), but CMB also has been used for estimating modern recharge rates, including those The chloride mass balance (CMB) method has been used extenthat have increased in response to land-use change, spesively to estimate recharge in arid and semiarid environments. Required data include estimates of annual precipitation, total Cl Ϫ input cifically where vegetation had been altered and deep-(from dry fallout and precipitation), and pore water Cl Ϫ concentrarooted trees were replaced with shallow-rooted grasses tions. Typically, CMB has been used to estimate ancient recharge, (Jolly et al., 1989; Walker et al., 1991). Because of its but recharge from recent land-use change has also been documented. simplicity, CMB is an attractive method. With properRecharge rates below a few millimeters per year are reliably detected assumptions about average annual precipitation and Cl Ϫ with CMB; however, estimates above a few millimeters per year apinputs, the only direct measurement required is the volpear to be less reliable. We tested the CMB method against 26 yr of ume-averaged Cl Ϫ concentration in the pore water. drainage from a 7.6-m-deep lysimeter at a simulated waste burial The general principles of the CMB method have been ground located on the Department of Energy's Hanford Site in southknown and practiced in irrigation management for years eastern Washington, USA where removal of vegetation has increased (USDA, 1954). In a manner similar to the CMB method, recharge rates. Measured drainage from the lysimeter for the past 26 yr averaged 62 mm yr Ϫ1 . Precipitation averaged 190 mm yr Ϫ1 with an salt mass balance in irrigation water is often expressed estimated Cl Ϫ input of 0.22 mg L Ϫ1 . Initial pore water Cl Ϫ concentraas the leaching requirement (LR), defined as the applied tion was 88 mg L Ϫ1 and decreased to about 6 mg L Ϫ1 after 26 yr, water volume (Q) times the input salt concentration while the drainage water Cl Ϫ concentration decreased to Ͻ1 mg L Ϫ1 .(C i ) divided by the drainage volume (D) times the salt A recharge estimate made using Cl Ϫ concentrations in drain water concentration (C s ) of the drainage water; that is, was within 26% of the measured drainage rate. In contrast, recharge estimates using 1:1 (water/soil) extracts were lower than actual valuesby factors ranging from 2 to 8 or more. The results suggest that when recharge is above a few millimeters per year, soil water extracts can lead to unreliable estimates of recharge.

“…Numerous borehole‐based approaches have been developed for monitoring a variety of properties and processes over widely ranging depths in soils and consolidated rocks (Dahan et al, 2009; Faybishenko, 2000; Hubbell et al, 2004; Levitt et al, 2005; Rimon et al, 2007; Salve, 2011; Sisson et al, 2002; Tokunaga, 1992). However, floodplains in mountainous regions commonly contain cobbles that make borehole instrumentation difficult.…”

Section: Site Descriptionmentioning

confidence: 99%

Deep Vadose Zone Respiration Contributions to Carbon Dioxide Fluxes from a Semiarid Floodplain

Tokunaga

Kim

Conrad

et al. 2016

Core Ideas A significant fraction of CO2 fluxes from a semiarid floodplain originates from below the root zone. Measured and calculated field CO2 fluxes are consistent. Laboratory‐measured respiration rates are consistent with field results. Although CO2 fluxes from soils are often assumed to originate within shallow soil horizons (<1‐m depth), relatively little is known about respiration rates at greater depths. We compared measured and calculated CO2 fluxes at the Rifle floodplain along the Colorado River and measured CO2 production rates of floodplain sediments to determine the relative importance of deeper vadose zone respiration. Calculations based on measured CO2 gradients and estimated effective diffusion coefficients yielded fluxes that are generally consistent with measurements obtained at the soil surface (326 g C m−2 yr−1). Carbon dioxide production from the 2.0‐ to 3.5‐m depth interval was calculated to contribute 17% of the total floodplain respiration, with rates that were larger than some parts of the shallower vadose zone and underlying aquifer. Microbial respiration rates determined from laboratory incubation tests of the sediments support this conclusion. The deeper unsaturated zone typically maintains intermediate water and air saturations, lacks extreme temperatures and salinities, and is annually resupplied with organic carbon from snowmelt‐driven recharge and by water table decline. This combination of favorable conditions supports deeper unsaturated zone microbial respiration throughout the year.