Shallow aquifer systems in surficial Quaternary sediments are geometrically complex, with highly variable hydraulic characteristics. Consequently these systems are extremely difficult to assess hydrogeologically using conventional investigation techniques and are often poorly understood. In a 500 km2 area of southern Ontario, hydrochemical techniques were used to investigate the hydraulic integrity and regional flow behaviour of 14 aquifer systems defined within 100 m of Quaternary overburden. This type of approach had been used successfully in more extensive bedrock aquifer systems but had not previously been applied on a regional scale to shallow Quaternary systems where sediment geochemistry and flow conditions are highly variable and rarely known. The study involved analysis of over 260 well waters for pH, major ions (Ca, Mg, Na, K, HCO3, Cl, and SO4) and subsidiary ions (NO3, I, Br, and F). Selected samples were analyzed for tritium.The groundwaters are classified into eight water types, two of which are sodium chloride in character and of bedrock origin. Of the remaining six, four are related by a chemical evolutionary sequence showing transition from a tritiated (> 30 TU), low-iodide (< 5 μg/L) CaHCO3 recharge water to an ion-exchanged, NaHCO3 water low in tritium (< 15 TU) and enriched in iodide (> 15 μg/L). The existence of this sequence implies a regional, composite flow system involving seven supposedly discrete aquifer systems.The two remaining water types are recent tritiated waters (> 20 TU) contaminated by road salt (Cl > 40 mg/L) and agricultural nitrate (NO3 > 10 mg/L). Together with the CaHCO3 waters these clearly delineate major areas of recharge and show that the covering of Halton Till is extensively permeable.