Recharge systems for protecting and enhancing groundwate

780
TOPIC 7
Sustainability of managing recharge systems / MAR strategies
other factors such as landfills and engineering structures (multi-storey building basements and underground
railway tunnels) may locally limit the groundwater level draw-up cone at locations adjacent to the pumping borehole
(Jones et al., in press). Borehole recharge capacity was calculated from Equation 5.
RC = (GWLMax - RGWL) * S c Eqn. (5)
Where RC is the recharge capacity, S c is the specific capacity of the borehole, RGWL is the rest groundwater level
and GWLMax is the maximum allowable head (i.e. the ground surface level). Borehole abstraction capacity is also a
difficult parameter to calculate because the minimum allowable head is limited by a number of factors that change
across the study area. These include existing borehole pump depths (derogation issues); minimum groundwater
levels acceptable to regulators that are yet to be defined; and local water quality issues associated with dewatering
the Basal Sands (a leaky layer that includes the Thanet Sands at its base). However as significant borehole hydraulic
tests are available from the SLARS area, abstraction capacity was determined from test results and values contoured
in the GIS accordingly. Alternatively where borehole abstraction capacity data are not available, abstraction capacity
may be calculated using Equation 6.
AC = (RGWL – GWLMin) * S c Eqn. (6)
Where AC is abstraction capacity and RGWL and GWLMin are the rest groundwater level and minimum allowable
groundwater level respectively. However in order to use Equations 5 and 6, values for specific capacity are required.
In this study reciprocal specific capacity values were determined from Equation 7 below (Bierschenk, 1963).
1/S c = s w /Q = B + CQ Eqn. (7)
Where s w is the drawdown in a pumped borehole, B is the time variable linear well loss coefficient and C is the
non-linear well loss coefficient that is independent of time. Theis (1967) rearranged the Cooper and Jacob (1946)
equation to show reciprocal specific capacity as a function of the transmissivity and time, but it is clear from
Bierschenk’s analysis that the Theis equation describes only the linear loss component of the specific capacity identified
in Equation 7. This gives the equation for the linear loss coefficient B, (Equation 8). It should also be noted
that this equation assumes full penetration and an effective radius that is equal to the actual borehole radius. In the
dual porosity Chalk of the SLARS area, this normally occurs after elapsed pumping (or recharge) times of 100 to
1,000 minutes.
B = 2.3 log [(2.25 T t) / (r 2 S)] / (4 π T) Eqn. (8)
Mace (1997) showed that specific capacity is a function
of the non-linear loss coefficient, C, and transmissivity,
T. However C is also a function of T. The mathematical
function is most easily determined from best-fit empirical
data using available pumping test data. In the SLARS
study area, only data from boreholes with similar construction
that may be considered to conform to the
Thames Water standard for borehole construction were
used. This empirical relationship is shown on Figure 2
and is defined in Equation 9.
C (d2/m5)
1E-6
1E-7
1E-8
Transmissivity (m2/d)
10 100 1000 10000
1E-5
C = 0.0004 T –1.3625 Eqn. (9)
1E-9
Figure 2. Relationship of C to T
ISMAR 2005 ■ AQUIFER RECHARGE ■ 5th International Symposium ■ 10 –16 June 2005, Berlin