2.DATA ISSUES IN GROUNDWATER STUDIES
In hydrogeological studies, data can be classified into four levels on the basis of the degree of interpretation (Struckmeier & Margat, 1995):
- Basic Data such as the results of direct observations or measurements;
- Primary Data derived from the simple treatment and interpretation of basic data, such as contours drawn from point water level measurements;
- Secondary Data resulting from more complex treatment and investigation methods such as statistics, spatial analysis and numerical simulation; and
- Tertiary Data or interpretations and classifications used for decision-making (information).
Table 2.1 gives examples of this transition from raw observations to information, with a progression in the level of analysis and interpretation. Hydrogeological features relate to the geological medium (eg. the aquifer), groundwater features relate to the groundwater per se and anthropogenic features relate to structures which access groundwater, such as bores and shafts.
Table 2.1 Classification of groundwater data based on the degree of interpretation (after Struckmeier & Margat, 1995)
|
Hydrogeological Features |
Groundwater Features |
Anthropogenic Features |
|
|
Basic Data |
Location of observation point (x,y,z); characteristics of outcropping strata; depth of top or base of aquifer; aquifer thickness; aquifer characteristics |
Location of observation point (x,y,z); depth to groundwater; spring discharge; pH; conductivity; temperature; chemical analyses |
Location of well, bore, shaft (x,y,z); depth of well; well discharge; drawdown |
|
Primary Data |
Hydrogeological boundary; elevation of top or base of aquifer; isopachs or structural contours of aquifer |
Potentiometric contours; hydrographs; mean spring discharge; salinity contours; isotope analysis |
Position of screen relative to mean sea level; mean well yield; mean abstraction or injection; maximum drawdown |
|
Secondary Data |
Aquifer parameters eg. porosity, permeability, transmissivity; grain size analysis |
Boundaries of phreatic, confined, artesian groundwater; flow directions and velocities; groundwater divides; interactions between groundwater system and river; recharge and discharge fluxes |
Specific yield; artificial recharge |
|
Tertiary Data (Information) |
Accessibility; risk of drilling failure; possibilities of leakage; level of aquifer protection |
Groundwater quality; suitability; vulnerability; protection areas |
Sustainable productivity; mean abstraction per unit area; injection potential; pollution |
Basic data are the factual observations or measurements taken from the groundwater system. This includes depth to standing water level measurements, flows from springs, chemical analyses of groundwater samples and surveys of outcrops of recharge beds. Although bores are the main mechanism for sampling the groundwater system, measurements of other features such as groundwater outcrops (eg. springs, salinas) and geological outcrops (eg. recharge areas) are also important.
A hydrogeologist typically combines other information with basic hydrogeological data to interpret primary datasets. The interpolating of depth to watertable contours by comparing bore measurements with topographic contours, remote sensing and drainage lines is one such example. More sophisticated approaches using numerical simulations, spatial analysis or statistics generate secondary datasets. These include pump test analysis to estimate aquifer parameters like transmissivity, flow nets, spatial overlay to derive recharge, and numerical models to quantify the aquifer-river interaction. Finally, tertiary data interpretation enables the input of hydrogeological information and advice into the decision-making process.
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