Cartographic representation

Decisions on the cartographic representation, mainly the legend, the map scale, and a suitable topographic base map must be made in the preparatory phase of a groundwater contamination inventory (see paras. 4.1.3, 6.1.1, and 6.1.2). The legend should give maximum information and assist in the interpretation of the map, whilst still maintaining clarity. To be of maximum value, the legend used must be clear, concise, and complete (Vrba and Zaporozec, 1994). The user must not be required to guess what the symbols and/or colours on the map indicate. The colours used must be carefully chosen to permit legibility (par. 6.1.3(ix)). Experience has shown that distinctly different and less intense colours are generally more effective than confusingly similar shades or bright colours. It is important to print a sample of the map first and adjust the shades and brightness as needed. Cartographers are well trained in colour composition and should always be asked for advice, to avoid poorly composed maps being presented.

Results of a groundwater contamination inventory are presented on maps in the form of points, lines, and areas. Point symbols of a varying form, size, orientation, filling, and colour (Struckmeier and Margat, 1995) show, for example, the location of a well, its size, and its yield or the location of a buried storage tank or of another point source of contamination and its status.

Line symbols are used for linear features such as pipelines, streams, and boundaries. Areal characteristics, such as diffuse sources of contamination, extents of contaminated areas, or source protection zones, are illustrated by colours and ornaments (patterns). Symbols, colours, and ornaments (patterns) should be based on recommended international standards (Struckmeier and Margat, 1995; Vrba and Zaporozec, 1994) as far as possible.

C.2 Groundwater contamination inventory map

When designing a groundwater contamination inventory map, it is useful to categorise the pre-sented information into primary and secondary classes.

C.2.1 Primary information

The primary information presented on a groundwater contamination inventory map are the contamination sources and the ambient groundwater quality conditions, including the spread of

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existing contamination of groundwater. Symbols in the proposed legend have been adapted from the symbology developed by IAH for groundwater vulnerability maps (Vrba and Zaporozec, 1994) and for hydrogeological maps (Struckmeier and Margat, 1995). The proposed symbols for potentially contaminating activities, i.e. agricultural, municipal, mining, industrial, and other, are included in Table C.1. Generally, they are shown in red or black. The ambient groundwater quality conditions, such as the area of contaminated groundwater or seawater intrusion are listed in Table C.2, and usually are shown in colours or patterns.

In addition, contamination inventory maps should also include the objects that may be affected by contamination spreading from the existing and potential sources of contamination, which may need protection against such contamination. These objects include all water supply sources and related features. Also shown should be protection measures already in effect (e.g.

source protection zones) or other important features, such as significant recharge areas. Recom-mended symbols are included in Table C.3, and are shown on a map in violet.

C.2.2 Secondary information

The secondary information includes topographic features and major hydrological features. This information should be included as a base map upon which the primary information is shown.

A suitable topographic base forms the background of the contamination inventory map.

Background topographic features (roads, railroads, settlements, streams, etc) serve as orientation features for the map user. For these, a standard set of topographic symbols should be used. Topo-graphic information is shown in a grey tone.

Hydrological information cannot be shown in a great detail on the contamination inventory map. It should include only selected features that are relevant to the goals and objectives of the inventory. Too much detail would reduce legibility of a map. If there is a need for more hydrological information, it can be included as one of the supporting maps (para. 6.2) alongside the main groundwater contamination inventory map.

The major hydrological features, both groundwater and surface water, recommended for the inclusion on a contamination inventory map are listed in Table C.4. Groundwater features (for example, springs, cones of depression, or direction of groundwater flow) are shown on a map in violet. Surface water features (character of streams, stream/groundwater relationship, etc.) are shown in blue.

C.2.3 Sample map

An example of the groundwater contamination inventory map with the proposed legend is included in Chapter 6, para. 6.1.5 (Fig. 6.3).

C.3 Groundwater contamination risk map

The compilation and construction of a groundwater contamination risk map is described in para. 6.2.4. Information presented on a groundwater contamination risk map is shown again in colours and symbols (para. C.1). The base map for a groundwater contamination risk map should be the same as that for the groundwater contamination inventory map (para. C.2.2).

The basis of a groundwater contamination risk map is a combination of a groundwater vulnerability map (para. 6.2.2) and a groundwater value map (para. 6.2.3). These maps should be constructed using colours recommended in international legends for hydrogeological maps (Struckmeier and Margat, 1995) and for groundwater vulnerability maps (Vrba and Zaporozec, 1994).

Combination of these two maps expresses the degree of protection needed and the associ-ated urgency for the protection of groundwater. The resulting groundwater units are differentiassoci-ated by areal colours showing the degree of groundwater protection urgency. The greater the urgency, the brighter and more reddish the areal colours. This map is overlain by a contamination inven-tory map showing the contamination risk rating of contamination sources and contaminated areas.

Symbols showing contamination sources and contaminated areas are recommended in Table C.1 and C.2, respectively.

Contamination risk rating is given by an index (letter or number) indicating the high (HR or 1), moderate (MR or 2), or low (LR or 3) potential contamination risk. Risk of groundwater con-tamination is a combination of the probability of concon-tamination (combination of groundwater vulnerability and contamination sources) and the consequence of contamination (groundwater value rating). The risk assessment does not consider actual health risks due to the presence of a particular contaminant in groundwater. Rating of contamination sources can be done by any of the methods mentioned in Chapter 5 (paras. 5.3.2 and 5.3.3).

Areal categorisation of the risk is commonly arranged in a traffic-light pattern. Usually, three categories of groundwater contamination risk are shown on the map (Table C.5):

1) High risk (HR) - deep red colour.

2) Moderate risk (MR) - orange colour.

3) Low risk (LR) - light green colour.

An example of the groundwater contamination risk map is included in Chapter 6, para. 6.2.5 (Fig. 6.10).

Groundwater contamination inventory