Some years ago a research project to study the chemistry of the ground-waters of the Upper Morne L'Enfer, Erin and Mayaro Formations was begun by the University of the West Indies (UWI) in collaboration with the Universities of Alberta and Birmingham and the Water and Sewerage Authority (WASA). The objectives of the project were to examine the origins of the variations in groundwater salinity within the aquifers used for water supply in southern Trinidad. For instance, potable quality groundwater occurs to depths of about 800m below mean sea level in the Morne L'Enfer and Erin Formations of southwestern Trinidad, but elsewhere, in the Penal and Barrackpore regions, saline groundwater is found near the land surface. A similar situation exists at the WASA Cedar Grove well field near Mayaro. The hydrodynamic relationship between the saline and fresh groundwaters is important when attempting to assess the long-term yield of these aquifers. Nevertheless, given the hydrogeological complexity of the aquifers, the extensive drilling and test pumping required for evaluation was not feasible - even in the bright economic climate of the early 1980s. Consequently, a pilot project was proposed to study the trace element and stable isotope chemistry of the groundwater in order to examine whether these chemical techniques could be used to refine the regional hydrogeology previously defined by local studies and adhoc well-field development. Unfortunately, arrangements for sample collection and analysis collapsed and the project was abandoned. However, some water samples were collected from wells in the Mayaro Formation by UWI/WASA personnel and were analysed for stable isotopes by the stable isotope laboratory of the University of Alberta's Department of Geology. These preliminary results are presented here as they may be of interest to the environmental geoscience community in Trinidad and Tobago. These techniques are widely used in Venezuela and Brazil, (Tamers 1963; Salati et al., 1974), and there are laboratories for the determination of environmental stable isotopes in both of those countries.
The stable isotopes commonly used in hydrology and hydrogeology are deuterium and oxygen-18. Natural waters consist approximately of 99.8% H₂0-16, 0.2% H₂0-18 and 0.016% of H₂O-16, i.e. the oxygen-16, oxygen-18 and deuterium isotopes of these elements. The molecular weight of H₂0-16 is 18 whereas those of H₂0-18 and H₂O-16 are 20 and 19 respectively.
Because of the differences in molecular weight fractionation occurs in which the heavier isotope is concentrated in the less mobile phase, i.e. liquid rather than vapour. In natural waters fractionation is due primarily to evaporation, although isotopic changes can occur in thermal springs or in soil waters rich in carbon dioxide. Hence measurement of the ratio of the heavier isotopic species to the lightest can yield information on palaeoclimatology, sources of groundwater recharge and discharge and mixtures of groundwater of different origins. Measurement of the ratio is by atomic adsorption mass spectrometry and careful field sampling and preparation techniques are required. The concentrations of isotope ratios are reported relative to SMOW (Standard Mean Ocean Water), which is a standard water sample kept at the International Atomic Energy Agency in Vienna. Units are delta values in parts per million (0/00) defined as:

[^R sample - ^R SMOW]

Del = ___________ x 1000

^RSMOW

in which R refers to the measured ratio. Generally the mean isotopic composition of rainfall is strongly correlated with mean annual air temperature so that there is also a close correlation with latitude. Thus, at tropical latitudes the relative depletion of del 0-18 and del deuterium is less than at locations further from the equator. Also, the concentration of del-O 18 is correlated with del-deuterium on a global scale, although each may vary considerably in precipitation on the local scale. The relationship is expressed by the Meteoric Water Line as:

§D = §S0-18 + 10(0/00)

Deviations from the MWL are due commonly to relatively greater depletion in O-18 than D by evaporation.

The del-deuterium and del-O18 values listed in Table I for groundwater from wells tapping the Mayaro Formation are shown plotted on the MWL Figure 3. They all cluster around the MWL, indicating that they are waters of recent origin. It is likely that they represent water recharged to the Mayaro Formation from the ridge marking the crest of the Mayaro Anticline. Indeed, WASA Well Mayaro 4, which was flowing when the sample was collected, is sited in a local discharge area east of the Cedar Grove Fault and the neighbouring ridge is the most likely source of recharge.

The indication of deep, recent groundwater recharge gained from the stable isotope data further implies that the fresh groundwater in the on-shore part of the Mayaro Formation is different in age and origin from formation waters off-shore. The Mayaro Formation is overlain offshore by successively increasing thicknesses of the Palmiste and Erin Formations, (Leonard, 1983). The Mayaro and Palmiste Formations are of Pliocene age, while the Erin Formation was deposited during the Pleistocene.
Carr-Brown (1973) has shown from the evidence of forminifera that sea-level in the vicinity of Mayaro was at least 61 m lower than its present position prior to 11,500 years B.P. Hence it is likely that aquifers within the Erin and possibly the near-shore Palmiste and Mayaro Formations would have been recharged with meteoric water. Because of the slightly cooler climate the isotopic ratio of this water would show different depletions than the present day. Recharge may also have been derived from the Guiana Highlands of Venezuela via the protoOrinoco system. Infiltration of sea-water during the rapid, postglacial rise of sea-level would have made the off-shore groundwater increasingly saline. Thus, the groundwaters should display the isotopic signal characteristic of mixed groundwaters of different origins.
The implications of the sparse isotope data, and the known occurrences of fresh groundwater within the Erin and Morne L'Enfer Formations of southwestern Trinidad, are that a broadly similar hydrodynamic regime may have been in operation during the glacial period prior to 11,500 years B.P. Van Andel et al. (1954) in a study of the Gulf of Paria and the Columbus Channel suggest that these areas were above sea level at that time. The subsequent Holocene transgression which would have resulted in the formation of the southwestern peninsula of Trinidad would also have caused sea water intrusion into the coastal aquifers of the Erin and Morne L'Enfer Sands. An appropriate, but crude model of the hydrogeology would be a classical Ghyben-Herzberg lens on a large scale, distorted because of heterogeneities caused by the geological structure of the peninsula.
A possible approach to resolving some of the ambiguities in this hypothesis would be to undertake stable isotope and other hydrochemical studies of the Erin-Siparia Basin. These studies could indicate whether the shallow and deep, fresh groundwater are of different ages and origins or whether they form a homogeneous mixture with sea water near the coast and underlying brines inland. These problems are not merely ones of academic speculation. The aquifers of southern Trinidad are heavily exploited for municipal, industrial and oilfield water suppliers. In this context it would be useful for development of the resource to know whether the concept of modern, local flow systems superimposed on a deeper, stagnant or slowly moving older system is valid. If this is true, then the deep groundwater would not receive significant volumes of recharge, and is probably in meta-stable equilibrium with underlying brines. Hence, an operating policy for the deeper aquifers would have to consider the nonrenewable nature of the resource and the possible constraint of upcoming of brines in response to localized over-pumping.
In conclusion, the few data available from the initial sampling phase of the project have opened the prospect of a deeper understanding of the hydrodynamics of Trinidad's 'Southern Sands aquifers by means of hydrochemical studies. This understanding, in turn, would lead to a more rational development of the nation's water resources and an advance in the knowledge of the regional hydrogeology of northeastern South America.

REFERENCES

CARR-BROWN, B. (1973). The Holocene Pleistocene contact in the offshore area east of Galeota Point, Trinidad, West Indies. Memorias VI Conferencia Geologica del Caribe, Margarita, Venezuela, pp381.397

LEONARD, R. (1983). Geology and hydrocarbon accumulations, Columbus Basin, Offshore Trinidad. Amer. Assoc. Petrol. Geologists Bull., Vol. 67(7), pp108l­1093.

SALATI, E., MENEZES-LEAL, J., CAMPOS, M~M. (1974). Environmental isotopes used in a hydrogeological study of north-eastern Brazil IN: Isotope Techniques in Groundwater Hydrology Vol. 1, Proc. Symp., IAEA, Vienna, pp259-283.

TAMERS, M.A. (1963). Surface-water infiltration and groundwater movement in arid zones of Venezuela. IN: Radio-isotopes in Hydrology, Proc. Symp. Tokyo, 1963, IAEA, pp339-353.

VAN ANDEL, TJH., POSTMA, H. & KRUIT, C. (1954). Recent Sediments of the Gulf of Paris. (Reports of the
Orinoco Shelf Expedition, Vol.2), K. Ned, Akad. Wetensch. Verh. Ser. 1, Vol. 20 (5),ppl-245.




ACKNOWLEDGMENTS: Leon Toppin of WASA assisted in the collection of water samples; Dr. Karlis Muelenbachs and James Freeman of the University of Alberta analysed the samples for stable isotopes.
Amoco Trinidad Oil Company (ATOC) and WASA permitted the sampling of water-wells at Tournebride 'and Cedar Grove, respectively.

EDITOR'S NOTE: Bill Milne-Home 1s a Senior Lee lecturer in Hydrogeology at the Centre for Groundwater Management and Hydrogeology of the Department of Applied Geology, at the University of New South Wales, Sydney, Australia. He was a Lecturer in Engineering Geology in the Department of Civil Engineering, UWI from 1979 to 1983, when the stable isotope project was begun. His interests include the application of chemical methods in hydrogeology and hydraulic testing of aquifers.

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