Nabarlek Pit decommissioning migration of sulphate, nitrate and radium ions in groundwater - preliminary modelling
Appleyard, S.
E-Publications; E-Books; PublicationNT; Report ; 41/1984
1984-04-01
Made available via the Publications (Legal Deposit) Act 2004 (NT).
Date:1984-04
English
Dept. of Transport and Works
Darwin
Report ; 41/1984
application/pdf.
Check within Publication or with content Publisher.
https://hdl.handle.net/10070/228496
https://hdl.handle.net/10070/674076
Technical Report WRD84041 Viewed at 14:07:09 on 29/07/2010 Page 10 of 34. I I I I I I I I I I I I I I I I I I I I I 4. FINITE ELEMENT MODELLING Due to a lack of hydraulic data for the fractured rock aquifer near the pit, finite element modelling was only carried out on a highly transmissive zone some 350m to the South east of the pit. Modelling was carried out on a 1150 x 600 m' consisting of 276 elements, each of dimensions 50 x 50 m2 (Fig 2.1). Standing water level data in this area indicate that the hydraulic gradient here is very low 2x10-3 ) and so initial potentiometric levels in the model were set to zero. Cooper Creek was represented as a line of nodes ~lith fixed potentiometric heads (Om) and seepage at a rate of 35 m'/day (A.G.C., 1984) I'laS assumed to be evenly distributed across five nodes. Pump testing in the area has indicated that the aquifer in this highly transmissive zone is confined and isotropic in nature. A value of 140 mO/day/rn is assumed for transmissivity and storativity is assumed to b 8 10-4 ]v - 1 1 ' r .: d - i h ~i r '.:..-e x _ _loee ~J.ng was ca_r.Le out us_ng t. e ~ __ "l,-e element programme SEFT~~N. Potentiometric contours for the model after 273 days of seepage are shm,-n in Fig 2.2. The model assumes no vertical recharge, and so the time period above was selected to give a stable solution which would represent the groundwater potentiometric surface after a single dry season of seepage. -1'he average pore water velocity of groundwater moving towards the creek after 273 days of seepage was 0.03 m/day, and if water consistently flowed at this rate it would take approximately 55 years for conservative solutes (such as sulphate ions) to travel across the gridded area. The extent of a hypothetical seepage plume 273 days after seepage is first detected in the modelled area is shown in Fig 4.3. Note that this is not the time elapsed since seepage from the pit commenced; AGC (1984) estimated groundwater velocities near the pit to be in the range 0.02 to 0.6 m/day, and so seepage from the pit ,vould take from 1 to 34 years to reach the modelled area. SA2/11 :TJ