Relation of Groundwater Quantity and Quality (Proceedings of the Hamburg Symposium, August 1983). IAHS Publ. no. 146.

Geochemical parameters as indicators for groundwater flow

P, FRITZ Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

ABSTRACT Groundwater flow in the subsurface and discharge into surface systems can be described and monitored with a variety of conservative and non-conservative chemical and isotopic tracers. However, no general rules concerning their applicability can be proposed although environmental isotope techniques are particularly well suited to identify and quantify groundwater runoff. However, in deep aquifers where it is normally not possible to apply physical techniques, the use of indirect geochemical/isotope tools is also not without problems. Indeed, their application is not valid where in situ modification of chemistries occurs because of intensive, long term rock-water interactions. This is the case for deep and very saline groundwaters found at depth throughout the Canadian Shield. Where chemical or isotopic variations are observed, they are due to mixing with shallower groundwater, often in response to man's activities.

Paramètres géochimiques utilisés comme indicateurs de 1'écoulement des eaux souterraines RESUME L'écoulement de l'eau souterraine dans le sous-sol et ses apports au système des eaux de surface peuvent être décrits et surveillés grâce à divers traceurs chimiques et isotopiques conservatifs ou non conservatifs. Cependant on ne peut pas proposer de règles générales concernant leurs possibilités d'application bien que les techniques des isotopes de l'environnement soient particulièrement bien adaptées pour identifier et quantifier l'écoulement souterrain. Cependant dans les aquifères profonds où dans des conditions normales il n'est pas possible d'appliquer des techniques physiques, l'emploi des techniques indirectes géoçhimiques/isotopes n'est pas sans poser des problèmes. En effet leur application n'est pas valable là où se produisent in situ des modifications de la composition chimique par suite d'interactions intenses et à long terme entre la roche et l'eau. Ceci est la cas pour des eaux souterraines profondes et très salées que l'on trouve dans le bouclier canadien. OÙ des variations chimiques ou isotopiques sont observées, elles sont dues à un mélange avec des eaux souterraines peu profondes, souvent par suite de l'influence des activités de l'homme.

229

230 P.Fritz

I PRODUCT I ON

In many hydrogeological systems, groundwater flow is most easily described in terms of time dependent changes in tracer constituents which occur in response to variable tracer input from a point-source (e.g. a landfill) or through rock-water interactions. However, one has to remember that the dissolved constituents of most groundwaters are not in chemical or isotopic equilibrium with the aquifer minerals and, therefore, over time geochemical and isotopic changes may also occur in stagnant groundwater systems, i.e. the geochemical evolution of a groundwater can occur in space and/or time. It is mainly the former which is of interest if groundwater flow is to be recognized.

This presentation addresses some aspects of problems associated with the tracing of groundwater flow and considers the use of environmental isotopes as well as inorganic dissolved compounds as tracers in shallow and deep systems.

GROUNDWATER MOVEMENT IN SHALLOW SYSTEMS

Ideal tracers for water movements are those which are incorporated into the water molecule, i.e. the isotopes of oxygen and hydrogen. Their concentrations in normal groundwaters are not affected by geochemical, biological or physicochemical processes; they display a conservative geochemical behaviour. Of special significance is

3

tritium ( JJ) whose abundance is not only determined by the variable input which has occurred during past decades (see Moser & Rauert, 1984) but also by its decay (tj m = 12.43 years) which introduces time-dependent variations and makes it an ideal tracer to monitor and describe recharge environments.

An interesting demonstration for the delineation and dating of groundwater flow from a recharge area was presented by Egboka et al. (1983) for a landfill site at Camp Borden, Ontario, where a leachate plume entered a sand aquifer (Fig.l). The site was monitored through a network of piezometers and analyses on samples collected include major ions, trace-elements and environmental isotopes. The leachate plume can be delineated by chloride (Fig.l) which shows that the front has migrated approximately 700 m since about 1940 when the site was first established. Tritium analyses date the plume's history for the past 25 years or so and provide the basis for contaminant and groundwater flow modelling with special emphasis on recharge and dispersion.

The use of conservative tracers such as chloride or tritium to recognize migrating plumes is an essential prerequisite for studies of the contaminant hydrology of landfill sites. There, and in many "normal" groundwater environments, the concentration of most ions is not only dependent on the input from a "source" but even more so on the changing geochemical environment within flow-systems (Schulz & Reardon, 1982). In a landfill environment, this differential behaviour provides the framework against which geochemical changes in the system can be measured. If, in addition it is possible to use 0 or H contents in the water as a "tracer" (e.g. Matthess et al., 1976), then the attenuation of contaminants can be discussed in terms of purification vs. dilution, i.e. the active removal of

Geochemical parameters and groundwater flow 231

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contaminants from a leachate vs. their simple dilution through mixing with uncontaminated groundwaters.

Monitoring of recharge at landfill sites is a relatively simple task but presents a certain challenge in "normal" environments such as a research site at Pinawa, Manitoba, Canada (Egboka, 1980). This site is typical for most sandy recharge areas in pro- and postglacial deposits in Canada and elsewhere where the geochemical characterizat­ion of groundwaters occurs already within the unsaturated zone and a fluctuating water table environment (Reardon et al., 1980). Typically, calcite saturation is reached and subsequent geochemical

232 P.Fritz

changes in the recharge environment are minimal. Thus, the migrating "plumes" of infiltrating groundwater can only be mapped by tritium. 0 and H analyses are of no use because their abundance throughout

the system is very close to the average annual isotopic composition of local precipitations.

Mozeto (1981) attempted to use radiocarbon in delineating and "dating" recharge plumes in this system but showed that its concentrations were affected by geochemical reactions and isotope exchange and could only indicate approximately the direction of flow.

GROUNDWATER DISCHARGE FROM SHALLOW SYSTEMS

The "Vorfluter" for both small groundwater systems, discussed above, are local streams. A few comments on discharge from subsurface to surface systems are thus appropriate. In systems which are not under hydraulic stress following, for example, precipitation events, baseflow discharge will have the chemistries of local shallow flow systems. It is then possible to identify those geological units which are the main contributors of groundwater (Schwartz & Milne-Home, 1982). Following major rainfall or snowmelt events, however, the response of the groundwater system is quite different and discharge volumes are strongly affected. Both chemical and isotopic techniques have been used to separate runoff hydrographs into contributing components (Fritz et al., 1976) and several major observations are possible:

(a) Chemical constituents such as Ca, Mg, Na or CI are not "conservative" because rainfall will acquire a dissolved load before it enters a stream. Thus, chemical characteristics which distinguish rain from groundwater are lost and hydrograph separation is no longer possible (e.g. Drever, 1982, pp. 186-190).

(b) If the rainfall or snowmelt is isotopically different from groundwater then l 8 0 , 2H or 3H can be successfully employed to quantify direct runoff and groundwater discharge. Under favourable circumstances, water flow from the unsaturated zone can also be assessed (Sklash & Farvolden, 1979).

(c ) All chemical and isotopic techniques show that groundwater is a major component in all runoff studies based on hydrograph separations using environmental isotopes. These include snowmelt events (Hermann et al., 1981; Rodhe, 1981). Figures 2 and 3 show hydrograph separat­ions using stable isotopes and document the prevalence of groundwater discharge during peak runoff. As Matthess & Ubell (1983) point out this observation is contrary to the hydrograph separation techniques described in the hydraulics literature.

Increased groundwater discharge during runoff events is important if the chemical loading of the groundwater comprises toxic substances as is the case in many landfills and disposal sites. It signifies that the total runoff of contaminants increases despite dilutions which will occur because of mixing between groundwater and rainfall/ snowmelt. Consequences are evident if the discharges terminate in lake systems.

To explain the increasing groundwater runoff following precipitat­ion or snowmelt events, Sklash & Farvolden (1979) suggest the following: along the perimeter of discharge areas, the water table

Geochemical parameters and groundwater flow 233

13 14 15 16

MARCH 1978

FIG.2 Runoff hydrograph separation of the Lainbach, Lainbachtal, Bavaria. QR, QQ and QT are rain, groundwater and total runoff respectively (after GSF Jahresbericht 1978}.

and its associated capillary fringe are very close to the ground surface. Infiltration will convert the associated tension saturated capillary fringe into a pressure saturated groundwater ridge. A theoretical analysis of this effect has been published by Gillham (1984) and experimental studies to verify this assumption have been undertaken by Abdul & Gillham (1984) who confirmed the importance of

Total flow

FIG.3 Snowmelt and groundwater runoff in a northern drainage basin (after Rodhe, 1981).

234 P.Fritz

the capillary fringe for the rapid generation of runoff. "An experiment using chloride as tracer showed that groundwater discharge to the stream preceded surface runoff and that at early times, groundwater was the dominant component of stream flow."

DEEP GROUNDWATER SYSTEMS

Most large aquifer systems cannot be sampled and monitored in the same detail as was done at Camp Borden or Pinawa. As a consequence, recognition of flow becomes difficult. Yet, in many waste disposal programmes, it is necessary to document the absence of groundwater movements before disposal begins and indirect methods have to be used to evaluate either water ages or flow paths of groundwater systems. The approaches usually employ either radio-isotope analyses (see Moser & Rauert, 1985) or chemical parameters.

Radio-isotopes can provide the means to obtain water ages in situations where it is not possible to sample along a flow system. However, these tools are based on isotope abundance in dissolved constituents and the geochemical behaviour of these isotopes is usually non-conservative. Furthermore, radio-isotope concentrations may change with time not only because of decay or as a consequence of continuous rock-water interactions, but also because physical processes such as diffusion (Sudicky & Frind, 1981) or nuclear reactions (e.g. the subsurface production of 39Ar; Loosli, 1982). This introduces considerable uncertainty into the determination of absolute ages.

The greatest value of radio-isotope analyses lies, thus, in the determination of changes along flowpaths and if continuity can be demonstrated, then "age-differences" can provide information about groundwater velocity and residence times. This will require study of geochemical changes in the flow system and an understanding of the origin of the geochemical load. The distinction between the "acquisition" of an ever increasing dissolved load and geochemical modifications of a given load due to rock-water interactions within a flow system may be necessary.

Such distinction is possible if specific tracers can be found. As shown by Moser & Rauert (1985) the stable isotopes l 0 and H can label a water mass but their concentrations do not reveal much about the origin of a geochemical load. Major ion analyses and isotopic compositions of aqueous species may delineate evolutionary trends but they are not suited for detailed discussions unless monitoring and sampling at different points in a flow system is poss ible.

Considering this, is it then at all possible to use geochemical parameters to identify and quantify "flow" in deep aquifers? Probably not, but since geochemical data reflect the origin of dissolved constituents, indirect information about flow paths and water origin can be obtained.

A recent study by Lloyd et al. (1982) may be quoted as an example. It discusses the origin of saline and brackish waters in the British Chalk aquifer in north Lincolnshire. Comparison of Cl~ and I~ concentrations permitted classification of different groundwaters and indicated that different marine infiltrations had occurred. Figure

Geochemical parameters and groundwater flow 235

4 summarizes their observations. The Type I waters are considered to be very old (supported by low radiocarbon activities) and are characterized by iodide enrichments which are attributed to exchange with the chalk matrix. The effect of this exchange on the radio­carbon data has not been discussed, although it may not be negligible. Furthermore, this case constitutes an increased complexity in the geochemistry of iodide relative to general acceptance and as such is

MODERN SALINE GROUNDWATERS (TYPE H L — - - ^ ESTUARY WATER

2 3 4 5 6 7 8 9 10 II 12 13

crgr' FIG.4 Iodide and chloride concentrations in saline groundwaters from the northern Lincolnshire Chalk aquifer (after Lloyd et al., 1982).

consistent with recent studies by Behrens (1982). Nevertheless, in systems of this type where primary chemical features apparently are still recognized, radio-isotope analyses such as *ç determinations are usually meaningful, and in combination with chemical and stable isotope analyses, they can provide very detailed information on origin, flowpaths and geochemical evolution of a groundwater body.

However, if large scale equilibration between groundwater ana aquifer minerals takes place and if sampling access is limited, then the use of isotopic and geochemical tracers becomes very difficult. This is the case for deep and very saline groundwaters in the crystalline rocks of the Canadian Shield (Fritz & Frape, 1982). The total dissolved solids in these waters exceed 250 g 1~ and chemistries are dominated by Ca-Na-Cl. The Mg and K concentrations are low either because this is a primary feature and reflects past high-temperature events and/or because secondary minerals on fracture

236 P.Fritz

surfaces provide a ready sink. Sulphate and aqueous carbonate are very low, possibly as a result of gypsum and calcite precipitation.

The existence of these fluids has been known to miners and mining geologists for over 100 years who considered them to be residual ore-forming fluids. Yet oxygen and hydrogen isotope data carry a low temperature signature which could reflect low temperature equilibria between silicates and water. This could signify that these waters are very old, because such exchange is an exceedingly slow process. It has not been recognized elsewhere.

If this interpretation is correct, then these waters must be geochemically mature waters, possibly end-members of different evolutionary paths. We have attempted to discuss possible origins and the first question one must ask is whether the chemical load, not composition, is primary. At this point, the question cannot be answered, but it is interesting to study the concentrations of the most conservative compounds, a.o. chloride and bromide, and to compare the Br/CI ratio with Ca/Mg, these two ions being strongly affected by geochemical processes. This comparison is shown as Sudbury for the Canadian Shield in Fig.5 which includes data from two other saline groundwater systems in crystalline rocks, one at Stripa, Sweden, and the other on Vancouver Island, Canada (Saanich Peninsula).

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Co / Mg FIG.5 Bromide and chloride concentrations in groundwaters from crystalline rocks (after Fritz et al., 1983).

The most significant observation is that at none of these localities sea water ratios are found. The Vancouver Island samples were collected in a relatively short flow system (5 km length) in a granodiorite in which salinities increase to total dissolved solids in excess of 3000 mg l - 1. The origin of this load is not known, but isotope analyses document that the system is very young and that no

Geochemical parameters and groundwater flow 237

mixing with modern or fossil sea water is evident. Figure 5 shows Br/CI ratios to range from high values approaching that of sea water in shallow, non saline waters to 0.001 in the more saline waters. This change is almost certainly not due to a geochemical evolution but must reflect mixing of two salt sources. It is suggested that "hydrothermal" salts are present on grain boundaries, dead-end pores or micro-cracks which are now leached by passing groundwaters (Al-Mooji, 1982).

The above interpretation is similar to one proposed earlier by Nordstrom (1983) for the Stripa groundwaters , although there Br/CI ratios are much above those observed on Vancouver Island. The Ca/Mg ratios at Stripa are high and indicate rock-water interactions. Simple sea water intrusion even during early post-glacial periods is not totally excluded but geochemical changes have also taken place.

It is somewhat unlikely that the same interpretation is correct for both localities, although it appears to be the most realistic one. Unfortunately, background information is limited and to date no truly conservative parameter has been found or recognized to reconcile this difference.

The similarity between Stripa and Canadian Shield data (Fig.5) is interesting and one could speculate that both waters have followed similar evolutionary paths, although the chlorinity at Stripa does not exceed 700 mg 1~ . One also has to note, that not only the Br/CI ratio but also the concentrations of these ions in the Shield brines correspond to marine brines at the point of halite precipitation. However, possibly even more significant is the observation that all brine samples collected across the Canadian Shield fall on a straight line on a Br vs. CI diagram (Fritz et al., 1983). The most simple explanation for this observation is mixing between two end-member waters, one being a very saline brine and the other, normal, dilute shallow groundwaters. This would suggest that both bromide and chloride are indeed conservative in their geochemical behaviour and reflect the origin of the salt load -which does not have to have a marine origin although most magmatic/ volcanic environments produce Br/CI ratios below sea water values. Most other ions in these brines are controlled by local rock-water interactions (Fritz et al., 1983; Frape et al., 1983). Under these conditions, it is unlikely that any of the geochemical parameters can be used to map groundwater flows, and where variations occur, these can usually be attributed to mixing with shallow, less saline groundwaters. Active groundwater movements in the rocks of the Canadian Shield appear possible to a depth of a few hundred metres where water movement occurs along shear-zones and major fractures.

REFERENCES

Abdul, A.S. & Gillham, R.W. (1984) Laboratory studies of the effects of the capillary fringe on streamflow generation. Wat. Resour. Res., 20, 691-698.

Al-Mooji, Y. (1982) Saline groundwaters in the bedrock of the North Saanich Peninsula, Vancouver Islands, BC. MSc thesis, University of Waterloo, Waterloo.

238 P.Fritz

Behrens, H. (1982) New insights into the chemical behaviour of radioiodine in aquatic environments. In: Proc. International Symp. on Migration in the Terrestrial Environment of Long-lived Radionuclides from the Nuclear Fuel Cycle, (Knoxville, Tennessee, 1981), 27-40 IAEA Publ. SM 257-36, Vienna.

Drever, J.I. (1982) The Geochemistry of Natural Waters. Prentice-Hall. Englewood Cliffs, New Jersey.

Egboka, B.C.E. (1980) Bomb tritium in shallow sand aquifers. PhD thesis, University of Waterloo, Waterloo, Ontario.

Egboka, B.C.E. , Cherry, J.A., Farvolden, R.N. & Frind, E.O. (1983) Migration of contaminants in groundwater at a landfill: a case study. 3. Tritum as an indicator of dispersion and recharge. J. Hydrol. 63, 51-80.

Frape, S.K., Fritz, P. & McNutt, R.H. (1983) The origin and evolution of saline ground waters in the Canadian Shield. In: Proc. Fourth International Symp. on Rock-Water Interactions (Misasa, Japan, September 1983) and Geochim. Cosmochim. Acta, in press.

Fritz, P. & Frape, S.K. (1982) Saline groundwaters in the Canadian Shield. J. Hydrol. 36, 179-190.

Fritz, P., Al-Mooji, Y., Frape, S.K. & Salati, E. (1983) Saline ground waters in crystalline rocks, isotopic and geochemical considerations on their origin. International Conf. on Ground­water and Man, (Sidney, 1983).

Fritz, P., Cherry, J.A., Weyer, K.U. & Sklash, M.G. (1976) Runoff analyses using environmental isotopes and major ions. In: Interpretation of Environmental Isotope and Hydrochemical Data in Groundwater Hydrology 111-130. IAEA, Vienna.

Gillham, R.W. (1984) The capillary fringe and its effect on water-table response. J. Hydrol. 67, 307-324.

Hermann; A., Lehrer, M. & Stichler, W. (1981) Isotope input into runoff systems from melting snow covers. Nord. Hydrol. 12, 309-318.

Jahresbericht des Instituts fur Radiohydrometrie 1978 (1979) GSF Bericht R 207, Neuherberg, 79-89.

Lloyd, J.W., Howard, K.W.F., Pacey, N.R. & Tellam, J.H. (1982) The value of iodide as a parameter in the chemical characterization of ground waters. J. Hydrol. 57, 247-266.

Loosli, H.H. (1982) A dating method with 39Ar. Earth Planet. Sci. Lett. 63, 51-62.

Matthess, G. & Ubell, K. (1983) Allgemeine Hydrogeology. Ground-wasserhaushalt. Gebr. Borntraeger, Stuttgart.

Matthess, G.M., Fritz, P. & Brown, R.M. (1976) Deuterium und Sauer-stoff-18 als Indikatoren von Grundwasserverunreinigungen durch fest Abfallstoffe. Dtsch. Gewaesserkund. Mitt. 2, 37-43.

Moser, H. & Rauert, W. (1985) Determination of groundwater movement by means of environmental isotopes - state of the art. I: Relation of Groundwater Quantity and Quality (Proc. Hamburg Symp., August 1983), 241-257. IAHS Publ. no. 146.

Mozeto, A.A. (1981) Carbon isotope exchange in aqueous systems: a field and laboratory examination. PhD thesis, University of Waterloo, Waterloo, Ontario.

Nordstrom, K. (1983) Hydrogeochemical investigations at the Stripa test site. KBS Report on the Stripa Project, 83-Ol, Stockholm, Sweden, 37-54.

Geochemical parameters and groundwater flow 239

Reardon, E.J., Mozeto, A.A. & Fritz, P. (1980) Recharge in northern clime calcareous soils, soil water chemistry and carbon-14 evolution. Geochim. Cosmochim. Acta 44, 1723-1735.

Rodhe, A. (1981) Spring floods, meltwater or groundwater? Nord. Hydrol. 12, 21-30.

Schwartz, F.A. & Milne-Home, W.A. (1982) Watersheds in Muskeg Terrain, I and II. J. Hydrol. 57, 267-305.

Schulz, H.D. & Reardon, E.J. (1983) A combined cell/analytical model to describe two-dimensional reactive solute transport for unidirectional groundwater flow. Wat. Resour. Res. 19, 493-502.

Sklash M.G. & Farvolden, R.N. (1979) The role of ground water in storm-runoff. J. Hydrol. 43, 45-65.

Sudicki, E.A. & Frind, E.O. (1981) Carbon-14 dating of ground water in confined aquifers. Implications of aquitard diffusion. Wat. Resour. Res. 17, 1060-1064.