Summary This study is a review of the major and minor chemical constituents in groundwater in the Jurassic limestones of Gloucestershire, England. Particular emphasis has been placed on the identification of hydrogeochemical processes. Groundwaters in the limestones of the Great Oolite Series and the Inferior Oolite Series, where they are unconfined, are of the Ca, HCO 3 type. In areas where both limestone series are confined beneath clays there is a gradual change in the chemistry to a Na, HCO 3 type. Deep within the confined limestones, particularly in the Great Oolite series, an Na, Cl component has been identified. Natural heavy metal concentrations are generally very low except where old pipework has influenced the Pb or Zn content. Significant down-dip chemical changes in terms of redox conditions, pH, dissolved oxygen content, ion-exchange and the concentrations of minor constituents are exhibited by groundwaters in the limestones of both series. An oxidation-reduction zone has been defined in each limestone unit. In the Inferior Oolite the zone occurs some 8 km down-gradient from the onset of confined conditions. In the Great Oolite a similar zone occurs 1–2 km into the confined area. The delay in the onset of reducing conditions in the Inferior Oolite is tentatively attributed to leakage of Great Oolite water through the intervening Fullers Earth.

The measurement of water levels in the ground is a routine part of groundwater resource assessments and most site investigations. However the techniques used are often inadequate to reveal the true behaviour of aquifers, often resulting in inadequate data or a large number of expensive but useless readings. Continuous recording of groundwater levels using both float-operated chart recording and electric pressure transducers have been made on a wide range of observation wells to determine the most economical frequency of measurement for different aquifers and different purposes. The results of the investigation revealed a number of factors which are relevant in the observation of groundwater levels and these are discussed. Of these the construction of the observation point in the form of an open observation well, tube well, standpipe or one of a variety of piezometers is of fundamental importance. The behaviour of the aquifer as confined, or unconfined, with or without perched water tables, has considerable bearing on the results obtained. Barometric, tidal, fluvial, seasonal and abstraction effects on groundwater levels are discussed with reference to the amplitude of groundwater fluctuation and its rate of change. Finally suggestions are made for the design of observation points and the frequency of readings for different applications.

The geochemistry and nature of the flow of ground water not only control the supply potential but constitute clues to the whole geology of an area. A study has been made of the largest available assemblage of data from 161 wells for the Island of Montreal collected by the Geological Survey of Canada in 1951–53. Data indicated that the system is generally subartesian, flowing from the principal topographically high areas towards the shores of the Island. As the probable use is about 13% of the estimated recharge of 140 million liters per day, most wells could be supplied by local recharge. The study has confirmed the predominance of calcium bicarbonate ground water from the carbonate sequence. The waters appeared to be saturated with respect to CaCO3 in all but 10 wells. The presence of other types of waters suggests the effects of the igneous intrusions of the area, the post‐glacial marine submergence and the upward movement of waters from deep sources through fault and other structural zones. Confirmation of the significant variations in chemical composition in some neighboring wells indicated the future need for repetitive sampling from specific horizons for chemical and isotopic analyses.