Infiltration and underflow near Halkett in the braided Waimakariri Riverbed, New Zealand

Abstract

Field observations on a relatively low infiltration capacity of river channels led to the premise that significant recharge of groundwater at Halkett is, in part, from underflow. To check the field observations, a laboratory and field study was undertaken of infiltration which confirmed that a low infiltration of the bed of river channels is caused by sealing of sediment, and by air entrainment in infiltrating water. The infiltration rate is as much as 10 times as high in sediment situated on inter-Channel islands. Therefore, outflow from river channels to underflow and to groundwater is directly related to river stage. The dye Rhodamine WT was found to be useable as a groundwater tracer. Its use in the field confirmed information gained from ground surface-, river-, and underflow gradients, namely that the rate of groundwater recharge from underflow can exceed the rate of feeding of underflow from the river channels at a low river stage.