Tracing of contaminated shallow groundwater in a

The area of investigation is situated at the edge of the Rhine River valley and the mountains of the Palatian Forest in the western part of Germany. Some decades ago, the drinking water was mainly provided by springs from the forest valleys (nitrate below 5 mg/L). Owing to the increasing population, wells were drilled to satisfy the water demand. The groundwater from the shallow, sandy aquifer located in the intensive farming area (wine, vegetables) typically has an increasing nitrate concentration — in the particular case of the Maikammer community up to about 120 mg/L nitrate (see Fig. 9.27). The task of this inves-tigation was to check the possibilities of minimizing the nitrate load in order to safeguard the independent, decentralized water supply of the community for the future.

Owing to the lack of deep piezometers, it was necessary to get more information from the well itself. A depth specific, low budget sampling technique with two pumps shows that below 80 m the water is almost free of nitrate and very low in CFCs and tritium as well. The mean inflow of nitrate rich water occurs at a comparatively great depth between 60 and 80 m below surface (well Rans 2). In this deep interval the environmental tracers (CFCs, tritium) also have significant concentrations, so the groundwater must contain post-1950 recharge. It is noteworthy that the CFC-11/CFC-12 ratio is different from the atmospheric input. A CFC excess, that is a concentration higher than atmospheric values, with rather high CFC-11 (> 50 pmol/L) and moderately

high CFC-12 (about 5 pmol/L) concentrations were found upstream in an old inoperative well, Rans 1, and also in a newly drilled shallow piezometer.

The CFC-12 and nitrate data at the various sampling sites fit nicely with a binary mixing approach with two characteristic end members having high and low nitrate concentration (see Table 9.1 and Fig. 9.28). These findings indicate that the nitrate and CFC-rich waters originate from the same shallow groundwater regime. The question arises as to what mechanism allows the penetration of shallow, nitrate-rich groundwater characterized by a CFC excess to the deep aquifer. Two causes seem possible: (a) geological ‘windows’

connect the shallow and deep aquifer layers; or (b) the old, inactive well Rans 1 has acted as a hydraulic window over the years.

As a consequence of these findings, the upper screens of the well Rans 1 were sealed to about 60 m below surface. This resulted in an immediate dropping of the nitrate concentration from 120 to about 60 mg/L. But the stable end member-2 nitrate concentration of less than 30 mg/L was not affected at

FIG. 9.27. Time series of nitrate concentrations in two wells in Maikammer, Germany.

this time. As the CFCs showed further excess values, it can be concluded that residual shallow water masses still play an important role in the deep aquifer system.

A confirmation of this theory is provided by the temporal variation of the CFC and nitrate concentration with pumping (following a resting phase of the well). In analogy to a relaxation process, the concentration of nitrate (and TABLE 9.1. CHARACTERISTICS OF END MEMBER WATERS

End member 1:

shallow groundwater (< 60 m below surface)

End member 2:

(> 60 m below surface) Anthropogenic influenced with:

⎯ CFC-12, CFC-11 conc. in excess

⎯ CFC-113 conc. near detection limit

⎯ Nitrate conc. about 120 mg/L

Natural properties:

⎯ CFC-12, CFC-11 low concentration

⎯ CFC-113 conc. near detection limit

⎯ Nitrate conc. about 10 to 30 mg/L

FIG. 9.28. Water mixing scenario and nitrate CFC-12 content of assumed end members.

CFCs) increases until reaching a steady state value of about 60 mg/L nitrate.

This behaviour is interpreted as the contribution of the residual water component increasing while the pump is working. Three pumping tests in the last two years show that the timescale of the relaxation process increases from a long term point of view. Owing to the natural groundwater flow and mixing processes, the contribution of the residual water masses decreases with time.

This explanation indicates that through the elimination of the hydraulic window (well Rans 1), the natural remediation of the deep aquifer leads to a long term decrease in the nitrate contamination.

Despite a low budget investigation and the lack of deep piezometers, this example shows that just a few CFC measurements are sufficient to detect an anomalous water component in a deep groundwater. Moreover, the results point to the basic hydraulic processes responsible for the contamination of the deep aquifer system. A knowledge of this mechanism in ideal situation even permitted the elimination of the deep penetration and the subsequent remediation of the deep aquifer system.

In general, these examples show that owing to the low detection limit and the almost ideal transport properties of the CFCs in groundwater, the

Davao Gulf 0 2 km

N

1

3 8 9 12 Toril

Dacoville Batulosa Tugbok

29 24

Talomo River Davao River Tamugan River

Lipados River

Davao City Jayapura

Jakarta Manila

Madang

FIG. 9.29. Map showing the study area and location of wells.

specific CFC excess is a sensitive indicator for identifying unfavourable water components in the subsurface. Besides drinking water management, such features are also useful for monitoring mineral water or brewing applications.

In addition, landfill–groundwater and surface–groundwater interaction are possible areas where similiar investigations could be of benefit. It seems that, especially in densely populated countries, the CFC excess is of similar importance to the CFC age dating tool under normal conditions.

9.14. EQUATORIAL LOW TRITIUM ACTIVITY REGION IN