Presented paper

IMWA2019 Students work

Sedimentation Rates In Two Acidic Pit Lakes: Implications For Riverine Flow-through As A Closure Strategy

Lund, Mark; Polifka, Jonas; Quintero Vasquez, Marilyn; Ramesseur, Ravish; Yangzom, Denchen
Mine Water and Environment Research Centre, Edith Cowan University, Australia

In Lake Kepwari (Collie, Western Australia) a former coal pit, a PITLAKQ model suggested that the major source of the acidity in the lake was from surface and pit wall erosion. After 2013, when the Collie River East Branch was allowed to flow through the lake, the rate of sedimentation and organic component likely changed. We have speculated that a lack of organic matter inhibits ecological development in pit lakes, and connecting a river to a pit lake is likely to increase organic matter within the lake. Therefore, we aim to determine the amount and source of sedimentation (organic and inorganic) in two co-occurring acidic pit lakes: Lake Kepwari with river flow-through and WO5H without. This research is important because it can be used to enhance pit lake water quality models and further explore river flow-through as a closure option.

Sedimentation in Lakes Kepwari and WO5H were measured in April 2013, September 2016 and October 2018 using sediment traps. On each occasion, a 50 m chain per lake, with three sets of replicated traps (3) located at 10 m, 25 m and 40 m from the surface (n = 9). Each trap was 0.98 m long, 80 mm diameter and sealed at one end. Upon recovery, tubes were mixed thoroughly, subsampled and then filtered on to 0.5 µm pre-weighed filter paper. Dry weight (60oC to constant weight) and loss on ignition (LOI; 550oC for 1 hour) were measured. Additionally, in 2016 and 2018 a benthic grab sample was collected (top 0.1 m) from five sites at <10 m, 10-20 m and >20 m in both lakes. Sediment was homogenized, dried and LOI measured as above.

Sediment LOI was not significantly (P>0.05) different between lakes or depths. However, the LOI in sediment traps was significantly different (P<0.05) between lakes and depths, with Lake Kepwari having overall higher LOI which also increased with depth. The LOI from sediment traps remained similar between 2016 and 2018 in WO5H but was lower in 2018 in Lake Kepwari. Sedimentation rates were not significantly (P>0.05) different between 2013 and 2018, depth or lake, and ranged from 150 to 675 mg/m2/day. River flow through increased the proportion of organic matter (LOI) sedimentation, without altering the total sedimentation rate. This increase in organic matter did not result in a measurable increase in sediment LOI. Although river flow substantially improved water quality in Lake Kepwari, the development of a true lake sediment remains slow.