TRANSIENT MODEL SIMULATION TO STUDY THE EFFECT OF RECHARGE AND EVAPOTRANSPIRATION ON THE HEAD IN HIGH DENSITY FRACTURED CLAYEY TILL

Increasing need for landfills by municipalities creates a need to understand the geology of the area in which the landfill is located. The landfill at Sioux Falls, South Dakota is located in a glacial till which has three distinct zones as follows: high-density fractured clayey till, low-density fractured clayey till and no fracture clayey till. The water flow is relatively insignificant in the low-density fractured clayey till and no fracture clayey till as the hydraulic conductivity is 10-10 m/s. Hence, the focus is on the uppermost high-density fractured clayey till, where the hydraulic conductivity averages several orders of magnitude higher. It is also understood from previous studies that lateral flow is insignificant. But vertical flow is relatively important, with evapotranspiration (ET) and infiltration playing a significant role. The transient model created helps in understanding all these process. Better understanding of water movement in the till will facilitate an efficient design for an environmentally sound landfill. The transient simulation was run for three consecutive years with each year comprised of fall, winter, spring and summer seasons, such that it covers the entire spectrum of recharge and ET values. Different simulations were done by combinations of three different years such as wet, dry and moderate. The recharge rate for these three years varies considerably (wet 2 x 10-10 m/s, dry 6 x 10-11 m/s, moderate 1.3 x 10-10 m/s). The ET values were different for different years and also for different seasons of each year. Evapotranspiration values ranges from 6 x 10-13 m/s to 1.9 x 10-10 m/s. The transient simulation of this area reveals that there is a vertical movement of water in the shallow high-density fractured clayey till. This is evident from the abrupt change in the head value due to the change in the recharge and also nullification of the recharge by ET during summer months. ET is the primary mode of discharge and especially very effective during the summer months. The transient simulation also shows that there is no influence on the head by the previous season. The recharge and ET act as an independent unit on a specific stress period only.