API Publ 4734:2005 pdf download.Modeling Study of Produced Water Release Scenarios
Numerical and analytical model simulation results suggest that large spills (100 and 10,000 barrels) will not cause unacceptable impairment of ground water quality if the depth of soil penetration is small (i.e. the release spreads over a large area) and the depth to ground water exceeds 3 meters. However, the results predict that most large produced water releases that occur over thin (< 3 meters), sandy vadose zones have the potential to cause unacceptable impairment of ground water quality. Although no simulations were performed for small releases (< 100 barrels), the results from this study can be used to infer that small releases that spread over the land are unlikely to cause unacceptable impairment to ground water quality when the depth to ground water exceeds 3 m. Release scenarios where a high-chloride-concentration produced water collects (e.g., within an unlined bermed area or a topographic depression) above a thin vadose zone are more likely to cause ground water impairment relative to releases with contrasting characteristics (e.g.; a low- chloride-concentration-produced water and a release that spreads in a thin layer over the land surface). Other release scenario characteristics (e.g., climate and depth to ground water) have relatively less impact on the maximum chloride concentration observed in a nearby downgradient monitoring well.
The modeling results show that chloride molecules from a produced water release will eventually migrate from the ground surface to ground water as long as there is a net downward flux to the water table. However, because the downward flux of chloride to ground water is often very small, close to zero in arid climates, the migration of chloride to ground water does not necessarily create material impairment of ground water quality. Simulated releases to a thin vadose zone in a humid climate with coarse textured soils result in the earlier arrival of the maximum concentration at an adjacent down-gradient monitoring well relative to a release with contrasting characteristics (e.g., a thick clay vadose zone, in an arid climate). Key release- scenario characteristics that have an impact on the time it takes for the maximum chloride concentration to be observed in the well are climate, soil type and depth to ground water.
Homogeneous vadose zone soil profiles are rare in nature. Therefore several scenarios with contrasting climate settings were analyzed to determine how clay layers intermixed with sand will affect chloride movement in the vadose zone and the subsequent impact on chloride concentration in ground water. Chloride concentrations simulated for a monitoring well down gradient of the release for the heterogeneous profiles decrease with increasing clay layer thickness in the vadose zone. The modeling results show that the increase in total thickness of clay layers in a profile slows down the chloride movement and results in lower concentrations in an adjacent down gradient monitoring well.