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 m3/month inflows to the lake

• Total of 429,000 m3/yr
• Total of 429,000 m3/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 oC winter 25 oC summer 25 C summer Maximum > 30 oC

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 > 30oC

Nutrients

• No evident

correlation between water level and

• ptimum 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

• btained.

Summer infiltration simulated

using Feb. 2008 lake level data – modified K’s fed back into calibrated model.

Two Models Calibrated

Lake Parameters Kh = approx. 15 m/d Kv= 0.04 to 0.06 m/d Kh = 1 to 5 m/d Kv= 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

Kh = 15 - 120 m/d

h

Kv = 0.1 Kh Sy = 0.2

Lake Jualbup

Kh = 1000 m/d and

1 - 5 m/d at boundary

Kv = 0.2 to 0.5 m/d Sy = 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

Assessment of Water Flow

Infiltration to Surrounds Infiltration Down Flow NE to SW Flow SW to NE Calibrated Model

• Flow to surrounds not

significantly reduced, particularly in summer

• Infiltration down

significantly reduced

• Flow to north-eastern

sector of the lake, where

Summer 61 239 25 5 Winter 155 740 61 61 75% Reduction Summer 67 142 226 Winter 99 230 504

sector of the lake, where it remains unsealed, is significantly increased

• Infiltration at the

north-eastern sector

• f the lake increases

from:

• 64 to 224 m3/d

in summer

• 319 to 860 m3/d

in winter.

Values are average over one year (2009) in m3/d

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 m3/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

• utflow.
• Recommend ongoing monitoring of groundwater and lake-

water levels and water quality

Questions?