Hydrogeological Investigation and Numerical Groundwater Modelling - PowerPoint PPT Presentation

Hydrogeological Investigation and Numerical Groundwater Modelling and Numerical Groundwater Modelling to Assess Management Requirements for the Maintenance of a Permanent Body of Water in the Lake

Presentation Outline � Scope of work � Background hydrogeology and lake water quality � Development of a numerical groundwater model � Results of modelling � Results of modelling � Conclusions and recommendations

Scope of Work � To investigate hydrogeological implications of maintaining an “adequate” body of water in Lake Jualbup through the application of a polymer additive to the lake bed to reduce permeability. to the lake bed to reduce permeability. � Consider reduction in permeability of 25, 50 and 75%* � Effect of raising the drain outlet? � Effect of infiltration via soakwells in Shenton Park catchment? � Consider water depth required for acceptable water quality and visual appeal * Information from supplier indicated product forms complete seal where applied and % reduction achievable only by modifying area of application

Current conditions at Lake Jualbup Current conditions at Lake Jualbup

Winter Groundwater Levels – September 2011 Lake Jualbup is primarily a • compensating basin, receiving stormwater runoff. On average it is estimated that • between 5,400 and 83,700 m 3 /month inflows to the lake Total of 429,000 m 3 /yr Total of 429,000 m 3 /yr • • In winter the lake acts as a • recharge point to the superficial aquifer

Summer Groundwater Levels – March 2012 In summer groundwater • flows south-south-west and at least part of the year the lake acts as a through flow lake. Water in the lake in • summer is generally an summer is generally an expression of the groundwater.

Water Quality Wetting-Drying Cycle � Lake water is fresh: � average 250 mg/L TDS � maximum 575 mg/L TDS � Temperature: � 15 o C winter � 25 C summer � 25 o C summer � Maximum > 30 o C � pH � Circumneutral – 6.8 � Range 4 to 9.5 � Dissolved Oxygen: � Frequently < 5 mg/L � Nutrients: � TN 0.07 – 4.2 mg/L (1.2 mg/L) � TP 0.02 – 0.69 mg/L (0.2 mg/L)

Field Parameters Salinity generally • higher during summer low water level Peak temperatures at • summer low water summer low water level Algal outbreaks have • been reported at times where T > 30 o C

Nutrients No evident • correlation between water level and optimum water quality Field notes report Field notes report • less obvious stagnation at times when water levels are above 4 m AHD

The Modelling Process

Rebuild Regional Groundwater Model: � RW model from 2009 reconstructed in Visual Modflow to enable modelling of lake drying and re- wetting. � Run-off to lake calculated for each month from 2008 based on Sim’s equation. equation. � Input as recharge to the lake. � Lake parameters and rainfall recharge and ET values modified until an acceptable fit with measured Lake and GW levels obtained. � Summer infiltration simulated using Feb. 2008 lake level data – modified K’s fed back into calibrated model.

Two Models Calibrated Lake Parameters K h = approx. 15 m/d K v = 0.04 to 0.06 m/d K h = 1 to 5 m/d K v = 0.2 to 0.5 m/d

Summer Lake Level Decline

Preliminary Modelling Results � Polymer additive covers 75% of lake 75% of lake bed from south-west to north-east.

Assessment of Infiltration Rates • Model 1 - most flow was sideways to lake surrounds • Decreased permeability in cells adjoining the lake to allow for wall and sloped lake bed • Recalibrated Model 2 predominantly downward flow Recalibrated Model 2 predominantly downward flow • Permeability of lake surrounds needed to be reduced further (new wall) to achieve desire water level rise. • Reduction of permeability in one area => increased infiltration rates in any area where permeability not reduced

Summer Lake Level Decline � model is most sensitive to background groundwater levels groundwater levels than changes to lake bed permeability

Calibrated Lake Levels Model Parameters: � Aquifer � K h = 15 - 120 m/d h � K v = 0.1 K h � S y = 0.2 � Lake Jualbup � K h = 1000 m/d and 1 - 5 m/d at boundary � K v = 0.2 to 0.5 m/d � S y = 1

Modelling Results Predictive • modelling assumes long- term average rainfall. New wall along • north, south and western borders Based on • monthly rainfall and run-off.

Comparison with Previous Lake Water-levels Water level falls • below desired minimum and eastern lobe dries out for all scenarios in scenarios in 2009 & 2010 Only the 75% • reduction in lake-bed permeability gives levels above minimum desired in 2011 & 2012

Flow to surrounds not • Assessment of Water Flow significantly reduced, particularly in summer Infiltration Infiltration down to Infiltration Flow Flow • Surrounds Down NE to SW SW to NE significantly reduced Flow to north-eastern Calibrated Model • sector of the lake, where sector of the lake, where it remains unsealed, is Summer 61 239 25 5 Winter 155 740 61 61 significantly increased Infiltration at the • 75% Reduction north-eastern sector of the lake increases Summer 67 142 0 226 from: Winter 99 230 0 504 64 to 224 m 3 /d • in summer Values are average over one year (2009) in m 3 /d 319 to 860 m 3 /d • in winter.

Summary of Key Results • Minimal impact on superficial aquifer (Fig. 19) • Outflow to ocean outfall drain estimated to increase 3 to 4 times (Fig. 21, Table 6) • Raising the drain outlet 0.3 m could reduce outflow from modified lake by 20% and increase late-summer water levels by a further 0.1 m (Fig. 22) • During an extended period of no summer rain an “adequate” water body would be maintained ~ 9 weeks longer, assuming long-term average winter rainfall. (Fig. 23)

Summary of Key Results • Approximately 340 to 430 m 3 /d could be required to augment lake levels during an extended summer interval with no rain (Table 7) • Reducing lake inflow through local infiltration wells: • 75% current inflow required to maintain desired minimum 75% current inflow required to maintain desired minimum water levels in summer (Fig. 24) • Could reduce outflow from modified lake by 40% or 60% with raised drain outlet (Fig. 25, Table 8) • Operational feasibility requires confirmation

Conclusions and Recommendations • To maintain an adequate body of water in Lake Jualbup, likely that 75 % of the lakebed will need to be sealed • If practicable, could start with 50% application to test and validate modelling results. validate modelling results. • Recommend raising drain outlet (with WC approval) 0.3m to increase lake storage, maximise infiltration and reduce outflow. • Recommend ongoing monitoring of groundwater and lake- water levels and water quality

Questions?