Section 5   Calculation of Groundwater Nutrient Loads

Nutrient load is a quantitative measure that has been used to estimate risk of eutriphication contributed from groundwater. Yu (1998) provided a method to estimate the groundwater nutrient loads.

Estimate of nutrient load discharged from groundwater into Wilson Inlet consists of two steps: the determination of groundwater discharge into the inlet; and the calculation of nutrient load into the inlet from the groundwater discharge.

5.1 Groundwater Discharge into the Inlet

The groundwater discharge rate was calculated based on Darcy's equation. This calculation is based on the difference in hydraulic heads between the two monitoring bores along each transect, the hydraulic conductivity, and the area of the seepage face. Figure 3 illustrates a cross section of an unconfined aquifer and the relevant parameters.

Department of Water
Wilson Inlet Groundwater Monitoring
Calculation of Groundwater Flux
Drawn: WY Approved: WY Date: 15/12/07 Figure: 3 Rev. A.
Job No. 42906500 File No. A4

Darcy's equation is expressed as

Q = W T I

whereQ = volume of seepage through a section of shoreline, in m3/day
W = width of the section, in m
I = hydraulic gradient
T = transmissivity of aquifer at the representative transect* for the section, in m2/day.

T can be calculated as

T = B K

whereB = thickness of saturated aquifer, in m
K = hydraulic conductivity of the aquifer, in m/day

I is calculated as

I = ( h1 - h2 ) / L

where(h1 - h2) = the hydraulic head difference between the two piezometers, in m L = horizontal distance between the two piezometers, in m

The representative transect* was selected to represent a section of the foreshore area within which hydrogeological conditions and land use activities are considered as being uniform.

5.2 Calculation of Nutrient Load

Nutrient load discharged from groundwater into the inlet is calculated based on the volume of groundwater discharged and the representative nutrient concentrations, expressed as

TNA = Q Cont 365 / 1000

TPA = Q Cont 365 / 1000

whereTNA = annual total nitrogen load, in kg /year;
TPA = annual total phosphorous load, in kg /year;
ContN = representative total nitrogen concentration*, in mg/L
ContP = representative total phosphorus concentration*, in mg/L

The representative total nitrogen concentration* was the total nitrogen concentration of a groundwater sample collected from a bore at a representative transect. This concentration is used as a representative concentration for a section of the foreshore area within which hydrogeological conditions, land use activities and nitrogen concentration in the groundwater are considered as being uniform. For this preliminary study, the concentration was calculated as the average of the two concentrations from each piezometer in each transect, or as the highest concentration in each transect.

It is recommended that the above method be used to calculate the nutrient loads from different land use areas.

Wilson Inlet Groundwater Nutrient Monitoring
URS Australia Pty Ltd

Draft Report

Prepared for Department of Water, 18 December 2007