Frequently Asked Questions - GWSDAT Data Entry

Time series groundwater solute concentration (in ng/L, ug/L or mg/L units)

Time series groundwater elevation (relative to a common ordnance datum)

Time series NAPL thickness

Well coordinates (in Cartesian coordinates, not latitude, longitude)

Yes, scalar X,Y well coordinates can be measured direct from a site plan. For example, by aligning a transparent grid of numbered squares with the N-S arrow on the site plan (north upwards) and reading off the relative X,Y locations of each well.

Yes, site plans in GIS Shapefile format can be imported as background images. The filepath to the Shapefile folder is entered in the third table of the Excel data input worksheet (entitled “GIS ShapeFiles”). The user can either enter the shapefile location manually or use the `Browse for Shapefile' function in the GWSDAT Excel menu for interactive file selection. Only the location of the main shapefile (file ending with a `.shp' extension) needs to be specifed in this table - the associated data files (e.g. .dbf, .sbn, .sbx, .shx) will be picked up automatically, provided they are in the same folder. It is possible to overlay multiple shapefiles up to a maximum of seven. Please refer to the GWSDAT user manual for additional information, including the conversion of CAD drawing layers to Shapefile format using ARC-GIS.

No, it is not possible to model vertical concentration distribution using GWSDAT. However, it is possible to group monitoring wells (e.g. by aquifer) and then plot each group separately. Multiple concentration values for a given solute at the same X,Y location, well group and sampling time are detected by GWSDAT and averaged prior to fitting of the spatiotemporal model.

In the event that a site- wide 3D interpretation of groundwater flow and solute transport is required we would recommended the use of numerical modelling software such as FEFLOW or MODFLOW. The considerable time and effort required to populate and run such complex models may be justified for high profile sites when working with high- cost 3 dimensional aquifer data.

The minimum input data requirements for GWSDAT to run correctly are as follows: For plotting of groundwater flow direction arrows:

• No solute concentration data required
• Minimum 3 well locations in coordinate table
• Minimum 1 measurement of groundwater elevation at each of these 3 wells within the user- selected model output interval

For plotting of groundwater elevation contours:

• No solute concentration data required
• Minimum 4 well locations in coordinate table
• Minimum 1 measurement of groundwater elevation at each of these 4 well locations within the user- selected model output interval

For plotting of solute concentration trends at individual wells:

• Minimum one solute: No groundwater elevation data required
• Minimum 1 well location in coordinate table
• Minimum 1 measurement of groundwater solute concentration at this well location

For fitting of valid spatiotemporal model and plotting of solute concentration contours:

• Minimum one solute: No groundwater elevation data required
• Minimum 3 well locations in coordinate table
• Minimum 2 concentration, time data points for each of these 3 well coordinates

In order to generate representative concentration contour plots, the spacing of monitoring wells needs to reflect the characteristic distance over which solute concentrations vary in the groundwater. This will vary from site to site: if groundwater flow rates are low or solute transport retarded then concentration hotspots are likely to occur and a closer well spacing will be required to map the concentration distribution. Conversely, if groundwater flow rates are high and solute transport is not significantly retarded then a larger well spacing may be adequate to map the concentration distribution.

Because the minimum well spacing required for effective concentration contouring varies from site to site, the user’s judgement is required in deciding whether the available data merits contouring. The presence of “redundant” data points that can be removed without significantly changing the concentration distribution is an indication that the monitoring well spacing is more than sufficient.

In the event that only a small number of wells (i.e. <4) are present, then GWSDAT v2.0 includes a circle plot option, which represents the data as circles coloured and sized to solute concentration, thereby avoiding the need to use potentially misleading concentration contours.

Similar arguments apply to the contouring of groundwater elevation data, although in the absence of significant topographic variation/ geological heterogeneity or groundwater abstraction/ water injection groundwater piezometric surfaces should be locally planar. The adaptive kriging algorithm used by GWSDAT to derive the piezometric surface requires a minimum of 4 well locations; flow direction arrows can, however, be generated for only 3 well locations.

GWSDAT handles non-detect data by a method of substitution. In accordance with general convention, the default option is to substitute the non-detect data with half its detection limit, e.g. ND<50ug/l is substituted with 25ug/l. Alternatively, non-detect data can be substituted with its full detection limit, e.g. ND<50ug/l is substituted with 50ug/l. Note that the entry of zero concentration values is not permitted.

During data analysis the user has the option to ignore the presence of NAPL when fitting the spatiotemporal model, or substitute detections of NAPL with site maximum solute concentrations. NAPL substitution should only be used if it is known that the solutes entered into GWSDAT are derived from dissolution of the NAPL. This functionality was introduced to avoid the situation whereby an area of wells containing NAPL appears as a minimum on concentration contour plots because groundwater solute concentration data is not available.

Note: Any solutes that are not derived from the NAPL can be excluded from the NAPL substitution process by flagging them as “NotInNAPL” or “E-acc” in the historical monitoring data table of the input worksheet. Note also that only one solute data point needs to be flagged to remove that solute from the substitution algorithm.