Optimising the effectiveness of sediments retention ponds for Waikato soil materials

Current Waikato sediment retention pond design is based on guidelines developed by the Auckland Regional Council. As soils in the Auckland and Waikato Regions are different, there is a need to investigate the effectiveness of sediment retention ponds in retaining sediments from Waikato soil materials. The objectives of this study were to: i) do a comparison between pipette, hydrometer and lasersizer methods for determining soil particle size and to characterise the sand, silt and clay in a range of Waikato soil materials, ii) evaluate turbidity and suspended solid concentration between the inlet and outlet of sediment retention ponds, and iii) investigate the use of chemical treatment (flocculants) in assisting sediment settling. Ten samples representing a range of Waikato soil materials were collected. Particle size was determined using hydrometer, pipette and lasersizer analysis. The pipette and hydrometer gave similar results. Lasersizer analyses were similar to pipette-hydrometer analyses for six samples. The remaining four samples analysed by lasersizer did not give a close agreement to conventional methods. However, error bars showed that between-sample variability was not large. The pipette was found to be the most reliable method for determination of particle size, however the lasersizer gave fast measurements which were easily repeatable. The soil texture of the ten Waikato soil materials tested ranged from sand to clay. A rain gauge connected to an autowater sampler was installed at the inlet of two sediment retention ponds, one at SH1 in Piarere and the other at a quarry in Ngaruawahia. Water samples were collected when rainfall reached 2mm in the previous 30 minute period. Samples were analysed for turbidity and suspended solids. The sediment retention ponds at both sites were effective, reducing suspended solids and turbidity by at least 94%. Water samples collected at Piarere showed a 94% reduction in turbidity (from 558.68 NTU to 35.27NTU) and a 97% declination in suspended solids concentrations (from 2365.63mg/L to 78.41mg/L). Results from water samples collected at Ngaruawahia demonstrated a 97% reduction in turbidity (from 491.33 NTU to 14.46 NTU) and a 95% drop in suspended solids concentration (from 210.43 mg/L to 9.5 mg/L). Flocculants (Polyaluminium Chloride, PAC) were being used at the sediment retention pond at Ngaruawahia. Further investigation into the effectiveness of flocculants in removing sediments from the water column found that samples 1 and 2 collected from the Ngaruawahia study site and allophanic soil materials do not require treatment with flocculants. The recommended dose of 8 ppm/litre of PAC was sufficient to treat sediment runoff without lowering pH level to a point that might induce aluminium toxicity in aquatic life of downstream rivers and streams for Ngaruawahia 3 and coarse materials of Hinuera Formation. PAC doses of 2.7ppm in the Hamilton Ash materials and 5.3 ppm in Hinuera Formation (fine materials) were sufficient to ensure flocculation. For the Piarere soil materials an 8 ppm PAC dose gave reasonable flocculation and 10.7 ppm PAC further reduced the turbidity after 24 hours