Ground Water Resources of the Stephenfield Watershed

There are a number of distinct aquifers found within the Stephenfield Lake watershed, with groundwater conditions being markedly different above the  Manitoba Escarpment versus below the Escarpment.

Above the Escarpment

 Above the Escarpment, bedrock is formed by the Odanah Member of the Pierre Shale.  This bedrock unit is highly siliceous and very hard, resulting in extensive fracturing.  Groundwater is able to move through the fracture network and, consequently, wells drilled into the aquifer will generally produce sufficient water for household or farm use.  Locally, well yields may be quite high with some wells in the aquifer (although not within the Stephenfield Lake watershed) reportedly being pumped at more than 100 gpm.  Groundwater quality is quite variable, ranging from fresh to brackish.  Shallow groundwater in the aquifer is often quite hard while deeper groundwater may be brackish with high concentrations of sodium and sulfate.  These brackish groundwater tend to be quite “soft”.  

 The Odanah Shale aquifer is overlain by glacial tills which contain layers or lenses of sand and gravel, forming local aquifers.  Water well logs indicate that these aquifers are widely distributed in the watershed above the Escarpment; however, their occurrence is largely unpredictable and test drilling may be required to define the extent and capacity of individual aquifers.  Where present, individual sand and gravel aquifers may provide significant yields; the village of Notre Dame des Lourdes being supplied by one such aquifer.  Groundwater quality is also quite variable, although generally fresh in most aquifers. 

Below the Escarpment

Below the Escarpment, bedrock consists of soft non-fractured shale and, further to the north and east, inter-layered shale, sandstone, gypsum and carbonate rocks.  The shale in these units do not form aquifers and, while the sandstones and carbonates do form aquifers, the groundwater is very saline.  Consequently, the only aquifers producing potable groundwater below the escarpment are formed by sand or sand and gravel bodies which occur in a variety of geological settings.  Four distinct sand or sand/gravel aquifers have been identified and mapped in some degree of detail.  These include the “beach”, “delta”, “spillway” and “silt/silty sand” aquifers (shown on Figure 1, after Wiecek and Render, 1998). 

At the end of the last glaciation, large volumes of meltwater were transported to Lake Agassiz through a deep channel eroded into the shale bedrock running roughly from the Tiger Creek drain to Treherne.  The channel is referred to as the Boyne River spillway, although it has largely been filled in with clay.  Where the water flowing through the spillway discharged into Lake Agassiz a large underflow fan (delta) of sand and gravel was formed.  These deposits form an important local aquifer termed the Delta aquifer.  As well, local sand and gravel deposits at the base of the spillway itself formed a second aquifer, termed the spillway aquifer.

The Delta aquifer forms an important local source of groundwater supply, providing water to the village of Treherne, a loading station, and rural residential and agricultural uses, including irrigation.  The saturated thickness of the aquifer approaches 70 feet in some areas allowing individual well production rates of as much as 100 gpm.  Groundwater quality is good to excellent in the eastern extent of the aquifer but is poor to brackish to the west.

There has been little groundwater development of the Spillway aquifer.  Permeable sand and gravel is only present locally and is obscured by a significant thickness of clay cover, requiring test drilling to find the sand and gravel deposits and determine their capacity.  Groundwater quality is generally poor to brackish with good quality water only in the Pellys Lake area.

Additional sand and gravel aquifers, in places covered by clay, are found in ancient beach deposits formed along the flanks and just below the Manitoba Escarpment.  These deposits were laid down where Lake Agassiz bordered the Escarpment.  In general these deposits are relatively thin but may have sufficient saturated thickness to provide a water supply for a rural residence or farm.  In the Rathwell area a more extensive beach deposit (the Beach aquifer) with a saturated thickness ranging from 5-25 feet has been mapped.  This aquifer is quite productive, supplying the village of Rathwell, a loading station, and providing water for agriculture as well as rural residential use.  Groundwater quality is good to excellent.

Farther to the east and north of the Escarpment additional aquifers are formed by fine sand to silt deposits which may occur as a relatively thin layer at ground surface or may be inter-bedded with clay.  The silt/silty sand aquifer mapped by Wiecek and Render is one example of these aquifers.  Groundwater production rates from these aquifers is generally quite low although may be sufficient for individual farm or rural residential supplies.  Large diameter wells are typically constructed into these aquifers with the storage capacity of the well providing water for peak use periods.  Groundwater quality is generally good although may deteriorate somewhat where the sands and silts are inter-bedded with clays. 

Groundwater Quality Issues 

 As discussed above, groundwater quality is quite variable across the watershed, and spatially within individual aquifers.  Groundwater is brackish to saline in some areas and, apparently, at depth within the Odanah Shale aquifer.  Where fresh groundwater is found, water quality issues may include elevated levels of hardness or undesirable concentrations of iron or manganese.  These problems can generally be dealt with using water conditioning equipment.

 A regional groundwater quality survey of rural wells in agro-Manitoba carried out by Manitoba Conservation in the late 1990’s found that about 40% of rural wells produced groundwater with detectable levels of coliform bacteria.  Microbial contamination is particularly prevalent in large diameter dug wells which are common below the Escarpment in the watershed.  Proper well maintenance is essential to reducing the occurrence of coliform bacteria in well water. 

Groundwater sampling within and adjacent to the watershed has also shown that groundwater from some wells may contain concentrations of nitrate, arsenic or uranium at levels above drinking water guidelines.  Elevated nitrate concentrations generally indicate contamination.  Typical sources of nitrate include septic systems, solid manure storage areas, livestock pens, or nutrient applications to fields.  Wells should be located as far as possible and up-gradient from potential contamination sources and constructed and maintained to reduce the potential for localized influx of contaminated waters. 

Elevated arsenic concentrations have been found in samples from some wells in the watershed.  Although comprehensive sampling has not been undertaken, current ideas are that the arsenic is naturally occurring and is most likely to be found in groundwater from sand/gravel aquifers within glacial tills above the Escarpment.  Uranium concentrations above the drinking water guideline have been found in groundwater samples from some wells drilled into the Odanah Shale aquifer.  The geochemistry of uranium occurrence is poorly understood. 

Reference

Wiecek, S. J. and F. W. Render, 1998.  Groundwater resources in the Upper Boyne River Watershed.  Report No. 98/01, Manitoba Conservation, Water Resources.

FIGURE: Generalized overview of the occurrence of aquifers in the Stephefield Lake watershed.

FIGURE: Fluid conductivity of groundwater samples overlaid on the aquifers map.  Groundwater quality is considered poor where fluid conductivity exceeds 2000 uS/cm.