Background

The GFV measuring system and the hydraulic conditions in monitoring wells

Porous Aquifers

Each monitoring well can be considered an anoomaly in the flow pattern of an aquifer. The flow resistance of filter gravel, screen and interior of the screen deviating from the sediment properties causes a deflection of stream lines (fig. 1)
Studies regarding the calculation of tream line deflection in wells are subject of various publications (e.g. BERGMANN 1970, 1971; DROST & HOEHN 1989; KLOTZ 1971; LORCH 1976; MOSER & RAUTERT 1980). There is a calculable regularity which shows the correlation of horizontal flow deviation with hydraulic resistance and well diameter.
This regularity is expressed in the factor alpha, which describes the relation between the inner diameter of the screen and the catchment area, which describes the relation between the inner diameter of the screen and the catchment area of the flowing groundwater in the well. KLOTZ (1978, 1990) developed tables depicting alpha values and calculations of the permeability of customary well screens.

The steamline deviation is refelected by the in-hole flow pattern. In general, the groundwater in the screen flows in a laminar way which results in stream lines. The regular symmetric formation of flow causes a stream line which passes the well in the radial center (fig. 2). At this position its direction corresponds to the flow direction of the groundwater around the well.

Determination of the Darcy velocity vf

BERGMANN (1970) proved in experiments and calculations that the flow velocity vH along the stream line in the center of the well via the alpha factor is directly proportional to the Darcy velocity vf:

Thus, under ideal conditions the Darcy velocity vf is determined by the flow velocity vH and the alpha factor. The alpha factor regards merely the flow verlocity, but not the flow direction. The flow velocity vH is extant only for a limited area of the inhole well (fig. 3).

Fig. 3: Schematic diagram depicting the alpha-value, the area where vH is valid, and the measurement area.

The flow velocity vH may be influenced by the following factors:

- Inhomogenities in the sediment surrounding the well, respectively along the filter gravel
- position of the screen slots with regard to the flow
- struts of the measuring probe
- stronly soiled screen slots

Pilot experiments using 4-inch screens DIN 4925 have verified that the position of the screen slots regarding the flow with velocities vH < 0,001 m/s has no effect on the flow direction. Components of the probe like the struts in the measuring section show a slight influence on the flow velocity vH, which can be calculated (SCHÖTTLER 1997).

The GFV-System is capable of optically checking the screen sctions in order to evade soiled slots while measuring or to take this into consideration at the evaluation of the data.




		Fig. 1: Distortion of groundwater stream lines in the vicinity of a fully screened well. Fow is directed from left to right (source: KLOTZ 1971)






	Fig. 2: Horizontal flow though a full sreeen(upper panel) and a half sreen (lower panel). Stream lines passing the center of the respective screens represent the prevailing flew direction (from left to right) (source: BERGMANN 1970).

Literature

BERGMANN, H. (1970): Über die Grundwasserbewegung im Filterrohr.- Diss. tech. Hochschule Graz 1968 - GSF_Bericht R 24: 211S.; GSF-Forschungszentrum für Umwelt und Gesundheit GmbH, München.

DROST, W. & HOEHN, E. (1989): Macrodispersivity in Granular Aquifers Determined with Single-well Techniques Using 82Br as a Tracer.-., Radiochimica Acta, 47; S. 13-20, 4 Abb; Verlag R.Oldenbourg; München.

- (1971): Hydromechanische Fragen zur Interpretation von Tracermessungen.- GSF-Bericht R 36: 29S., 13Abb.; GSF- Forschungszentrum für Umwelt und Gesundheit GmbH, München.

KLOTZ, D. (1971): Untersuchung von Grundwasserströmungen durch Modellversuche im Maßstab 1:1.- Geologia Bavarica 64: 75-119, 33 Abb., 7 Tab.; München.

. (1978): alpha-Werte ausgebauter Bohrungen.- GSF- Bericht R 176: 119S., 12Abb., 100Tab.; GSF-Forschungszentrum für Umwelt und Gesundheit GmbH, München.

. (1990): Berechnete Durchlässigkeiten handelsüblicher Brunnenfilterrohre und Kunststoff-Kiesbelagfilter (Stand 1990).- GSF-Bericht R 35/90: 55 S., 21 Abb., 14 Tab.; GSF-Forschungszentrum für Umwelt und Gesundheit GmbH, München.

LORCH, S. (1976): Sonde zur Messung der Geschwindigkeit und Richtung der Grund wasserströmung in einem Bohrloch.- Geol. Jb. E6; 7.Abb., S.67 - 79; Hannover.

MOSER, H., & RAUERT, W. (1980): Isotopenmethoden in der Hydrologie.- in: Matthess, G. (Hrsg.) : Lehrbuch der Hydrogeologie, 8: 397S., 227 Abb., 32Tab.; Berlin.

SCHÖTTLER, M. (1997): Meßbarkeit der Grundwasserbewegung durch Visualisierung der Strömung in Bohrbrunnen.- Diss. Univ. Köln; 119 S.; Shaker Verlag, Aachen.

Standards

DIN 4925: Filter- und Vollwandrohre aus weichmacherfreiem Polyvinylchlorid (PVC-U) für Bohrbrunnen mit Querschlitzung und Gewinde; Deutsches Institut für Normung e.V. (Hrsg.); Berlin.


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