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Many engineered barriers are expected to function for hundreds of years or longer. Over the course of time, it is likely that some barriers will experience infiltration to the point of breakthrough. This study compares the recovery from breakthrough of two storage–evapotranspiration type engineered barriers. Replicates of test plots comprising thick soil and capillary–biobarrier covers were wetted to breakthrough in 1997. Test plots were kept cleared of vegetation to maximize hydrologic stress during recovery. Following cessation of drainage resulting from the wetting irrigations, water storage levels in all plots were at elevated levels compared with pre‐irrigation levels. As a result, infiltration of melting snow during the subsequent spring overloaded the storage capacity and produced drainage in all plots. Relatively rapid melting of accumulated snowfall produced the most significant infiltration events each year during the study. Capillary barriers yielded less total drainage than thick soil barriers. By limiting drainage, capillary barriers increased water storage in the upper portions of the test plots, which led to increased evaporation from the capillary barrier plots compared with thick soil plots. Increased evaporation in the capillary barrier plots allowed more water to infiltrate in the second season following the wetting tests without triggering drainage. All thick soil plots again yielded drainage in the second season. Within two years of intentionally induced breakthrough, evaporation alone (without transpiration) restored the capability of the capillary barrier covers to function as intended, although water storage in these covers remained at elevated levels.

Scale effects have been observed between solute transport experiments performed in the laboratory and in the field. This study was conducted to quantify differences between laboratory scale (30 cm long) and field scale solute transport using 6 m long soil columns to represent field conditions. Steady state tritium, bromide, boron and chromium transport parameters determined from effluent breakthrough curves in small columns packed with homogeneous loamy fine sand were compared with those determined from breakthrough curves obtained from various depths in a large column packed with similar soil. The tritium and bromide transport parameters obtained from the large column were also compared to those obtained from the same large column during transient infiltration of solute containing water and to those obtained from a steady state experiment performed in a second large column packed with 29 alternating 20 cm thick layers of loamy fine sand and silty clay loam. The average tritium dispersivity determined in the large column filled with homogeneous soil was greater (4.53 cm) than that for the small columns (0.64 cm) and for the large layered column (1.38 cm). Dispersivities in the large columns appeared to be unrelated to depth. Sorption coefficients used to model boron and chromium transport also differed between small and large columns. Solute transport predictions for the large column made using sorption-related transport parameters determined in the small columns were good for boron but not for chromium. Steady state solutions of the convection-dispersion equation derived for homogeneous soils accurately described solute transport in the layered column. Solute pulses during transient infiltration lagged behind the wetting front. Transport parameters determined under steady state conditions accurately described transport under transient conditions. Transport parameters determined from small columns differ from those determined in large columns. These differences may be inconsequential for large scale predictions (based on results obtained on a smaller scale) for sorption-related parameters, but not for dispersivities. Unlike the small columns, the large columns allow the study of depth effects on solute transport which may be important in the interpretation of observations and the prediction of future events.

A coupled field-scale aquifer pumping and water infiltration test was conducted at the Idaho National Engineering and Environmental Laboratory in order to evaluate subsurface water and contaminant transport processes in a heterogeneous flow system. The test included an aquifer pumping test to determine the storage properties of the aquifer and the state of confinement of the aquifer (approximately) 190 m below land surface, and a vadose zone infiltration test to determine vertical moisture and radioactive tracer migration rates. Pump test results indicated that the Snake River Plain Aquifer was locally unconfined with a transmissivity ranging from 5.57 x 10(5) to 9.29 x 10(4) m2/day. Moisture monitoring with neutron probes indicated that infiltrating water was initially transported vertically through the upper basalt layer of the vadose zone, primarily through fractures and rubble zones, at an average rate of 5 m/day (based on vertical distance traveled and first arrival of water at the monitoring points). Analysis of breakthrough curves for a conservative tracer allowed estimation of the arrival of the peak concentration and yielded an average velocity of 1 m/day. The migration velocities from the neutron probe and tracer tests are in good agreement given the scale of the test and difference in analysis techniques. None of the data sets showed a correlation between migration velocity (arrival time) and distance from the point source, but they strongly indicate preferential flow through discrete fractures. Upon reaching the first continuous sedimentary interbed layer in the basalt formation, water flow was diverted laterally along the interbed surface where it spread outward in primarily three areas corresponding to topographic lows on the interbed surface, and slowly infiltrated into the interbed. The nonpredictable movement of water and tracer through specific fractures underlying the site suggests that a priori prediction of transmissive fractures in this media

Monitoring the movement of contaminants throughout the vadose zone requires the use of wells and the credibility of a monitoring program depends on obtaining an adequate seal between the well casing and borehole wall. The credibility of monitoring well installation was evaluated during the Idaho National Engineering Laboratory's Large Scale Pumping and Infiltration Test. Wells were drilled in and around a 6.5 acre infiltration basin with an air rotary rig using a downhole hammer, cased with PVC or steel, and the annular space back-filled with alternating layers of bentonite and sand. The purpose of completing the wells in this manner was to isolate fractured intervals (subvertical and subhorizontal interflow zones) in order to observe water movement during the infiltration test. Bentonite was used between sand intervals to prevent the borehole (or annular space) from serving as a conduit for vertical water flow or tracer migration. Neutron probes were used to confirm the presence and locations of each completion interval (sand or bentonite) by distinguishing differences in background-water content of the back-filling materials. Upon flooding of the infiltration basin with water containing radioactive tracers, water flow and tracer transport were monitored using neutron probes and an in situ, downhole gamma spectroscopy system, respectively. Results confirm that each well was installed correctly; water and tracers flowed through natural fractures in the subsurface and arrived at the monitoring sites located at sand completion zones. Significant water or tracer flow through the annular space between the well casing and borehole was not observed