Estimation of the preliminary groundwater reserve using numerical models

Abstract

English: The Water Act requires the determination of the Groundwater Reserve to secure the basic human needs and to protect the environment (aquatic ecosystems in terms of the Act). In other words, it is necessary to determine the allocatable groundwater resources and its use may not cause unacceptable negative impacts. As numerical models have been greatly developed in assessing groundwater resources in recent years, it is used to estimate the Groundwater Reserve. An investigation into existing groundwater numerical models has been done in order to select the suitable numerical model for determining the Groundwater Reserve. The MODFLOW program is selected for the present work because of the widespread of applications and the existence of user-friendly graphical interfaces, e.g. Chiang and Kinzelbach (in press). The procedure of estimating the allocatable groundwater resources can generally be divided into two steps: The first step is to estimate the total accessible groundwater resources by means of the regional groundwater balance. Groundwater fluxes, recharge, abstraction, and leakage to/from surface water bodies are balanced over the whole catchment. The methodology is used for the Pienaars River catchment, where the groundwater recharge is the only inflow term to the aquifer and its determination will directly affect the result. The interaction (water exchange) between groundwater and surface water (rivers and wetland) cannot be quantified without additional data, especially river flow rate and evaporation measurements in proper positions. To date, most available computer models are unable to compute the dynamic interaction between river stages and the groundwater levels. As a result, in most MODFLOW (or other groundwater models) applications, the river stages are often assumed to be known. This is, however, not applicable in many cases, where the change of the surface water profiles (river stages) due to groundwater or surface water abstractions must be predicted. A computer program which is able to couple the groundwater and surface water flow models, will be useful for computing and predicting both the groundwater and surface water levels and their interactions. The second step is to estimate possible negative environmental impacts due to groundwater abstractions. The estimated impacts are scales for decision-makers to give allowances to future abstractions. There are many possibilities to perform the second step, for example post-auditing or by using numerical models. The former is often not appreciated, because most groundwater systems are difficult or impossible to restore once they are damaged or contaminated. The latter is superior due to its predictive capability and its flexibility at the price of the data requirement. Because of the complexity of the saltwater intrusion processes, the program SEAWATER is developed to solve the steady-state salt-/freshwater sharp interface for using with MODFLOW. SEAWATER calculates the interface by means of an iterative process, which uses the Ghyben-Herzberg approximation to determine the location of the interface. The results of the program are compared with several analytical solutions.

Afrikaans: Die Water Wet vereis die bepaling van die Grondwater Reserwe om te verseker dat aan basiese menslike behoeftes en die bewaring van die omgewing (akwatiese ekosisteme m.b.t. die Wet) voldoen word. Die verbruikbare grondwaterbron, sonder onaanvaarbare negatiewe gevolge, moet bepaal word. Numeriese modelle, wat die afgelope tyd baie verbeter is en baie gebruik word in ondersoeke van grondwater bronne, is gebruik vir die beraming van die Grondwater Reserwe. 'n Ondersoek na bestaande numeriese grondwatermodelle is gedoen om 'n geskikte numeriese model te gebruik vir die bepaling van die Grondwater Reserwe. MODFLOW is gekies vir die studie omrede dit 'n wye toepassingsveld het en daar gebruikers-vriendelike grafiese koppelvlakke beskikbaar is, soos byvoorbeeld die van Chiang en Kinzelbach (in druk). Die metode vir die beraming van die verbruikbare grondwaterbron kan in twee stappe verdeel word. Die eerste stap is om die totale beskikbare grondwaterbron te beraam d.m.v. 'n streek grondwaterbalans. Grondwatervloei, aanvulling, onttrekking en suipelling van en na oppervlakwater word gebalanseer oor die hele opvanggebied. Die metode is gebruik vir die Pienaarsrivieropvanggebied, waar grondwateraanvulling die enigste invloei term na die akwifeer is. Die berekening van aanvulling sal dus die resultaat direk beinvloed. Die wisselwerking (water uitruiling) tussen grondwater en oppervlakwater (riviere en vleie) kan nie gekwantifiseer word sonder addisionele data, veral riviervloeitempos en verdamping by geskikte meetstasie liggings nie. Tot op hede kan die meeste rekenaarmodelle nie die dinamiese interaksie tussen riviervlakke en grondwatervlakke bereken nie. As gevolg hiervan word riviervlakke as bekend aanvaar in die meeste MODFLOW (of ander grondwater modelle) toepassings. Dit is in baie gevalle nie toepaslik waar 'n verandering in oppervlakwater (riviervlakke) a.g.v. die onttrekking van grondwater en oppervlakwater voorspel moet word nie. 'n Rekenaarprogam wat grondwater en oppervlakwater vloeirnodelle koppel sal handig te pas kom in die berekening en voorspelling van beide grondwater en oppervlak watervlakke en die wisselwerking tussen die twee. Die tweede stap IS om die moontlik negatiewe omgewingsimpak a.g.v. grondwateronttrekkings te beraam. Die geskatte impak kan gebruikword as maatstaf om toestemming te gee vir toekomstige onttrekkings. Daar is verskeie moontlikhede om die skatting te doen, bv. deur nabetragting of die gebruik van numeriese modelle. Eersgenoemde is nadelig in die sm dat dit in die meeste gevalle moeilik of ontrnoontlik is om beskadigde of besoedelde grondwaterstelsels te herstel. Laasgenoemde word verkies omdat dit 'n voorspellingseienskap het en buigsaam is t.o.v. kostes rakende data. A.g.v. die kompleksiteit van die soutwater indringingproses is die SEAWATER program ontwikkelom saam met MODFLOW die rustoestand skerp koppelvlak tussen sout-en varswater te bereken. SEAWATER bereken die koppelvlak deur middel van 'n herhalingsproses, wat gebruik maak van die Ghyben-Herzberg benadering om die ligging van die koppelvlak te bepaal. Die resultaat van die program word vergelyk met verskeie analitiese oplossings.