Water is often used when there is the potential for air and water pollution from dust traveling across the landscape or through the air. If water is used for dust suppression at the Hanford Site’s Central Plateau, infiltration of the dust-suppression water has the potential to provide a hydraulic driving force to further mobilize contaminants in the vadose zone and accelerate their arrival at the water table. The scope of this investigation is to use modeling analyses to estimate contaminant mobilization as a function of volume and rate of water addition, subsurface properties, and contaminant properties.
Four sets of simulations were conducted to investigate the impact of water application for dust suppression on solute transport in the vadose zone of the Hanford Site 200 Area and each set consists of several simulation cases. In first two sets of investigations (i.e., 200E and 200W), typical geologic profiles for the Hanford 200 East and 200 West Areas were used and contaminants were assumed to reside in the middle of the domain within the finer 10- to 20-m-thick layers. The third set of simulations (i.e., BY) investigated the impact of water applied for dust suppression on NO3 transport below the BY Cribs. The fourth set of simulations (i.e., S7) investigated the impact of applied water on I 129 transport below the S7 Crib. The main findings are summarized below.
For all four sets of investigations, the water for dust suppression creates water pulses at higher flux rates than the base case (i.e., no water application). As more water is applied, the peak aqueous flux rates are higher. The impact of the water pulses on solute transport is dependent on the distribution of the solute in the vadose zone.
When the solute is not distributed over a wide vertical range within the soil profile, the solute pulse arrives at the groundwater much later, as in the 200 East and 200 West simulations. In these cases, a larger volume of water translates to slightly earlier arrival times for the solute peak (up to 7 years) but has little impact on the peak flux rate and hence little impact on the solute concentration in the groundwater.
When the solutes are vertically distributed throughout the soil profile, as they are for the BY and S7 Cribs, the solute pulse arrives at the groundwater at nearly the same time as the water pulse, possibly because solute resides in nearly all of the profile and hence diffusion plays a minor role in solute transport. In these cases, the peak solute flux rate is much higher than the value of the corresponding base case, ranging from 12% to 167% for the BY Cribs and from 2.4% to 32.5% for the S7 Crib. Because the vadose water flux is only a small fraction (about 2% for the base case) of the groundwater flux rate, a higher solute flux rate because of applied water indicates a near proportionally higher solute concentration in the groundwater. Among the cases, the peak solute flux arrival time varies up to 100 years for BY Cribs and only 7 years for the S7 Crib.
When the same amount of water is applied, the variation of water application rate has very little impact on solute transport entering the water table.
Key Words: Hanford; Iodine; Nitrate; Central Plateau; 200 Area
Revised: December 12, 2019 |
Published: September 28, 2017