Proceedings - Teaching and Learning Centre - Simon Fraser ...

Fraser River Action Plan 3rd Research Workshop
Investigation into the Distribution of
Non-Point Source Nitrate in Two Unconfined Aquifers
J. Atwater
Civil Engineering
University of British Columbia
Most of the groundwater in unconfined aquifers in the lower Fraser Valley contains elevated levels of nitrate.
These aquifers are important sources of drinking water for many people; however, they also provide the base
flow for many of the tributary streams in the valley. Nitrate contamination of unconfined aquifers is not unique
to the valley but rather is found in almost every jurisdiction world wide. In a number of these locations there
have been reports of in situ denitrification. Included in these reports are local sites. The analysis of the
groundwater from three MOEPL wells located in the Abbotsford Aquifer suggest that denitrification was
occurring at depth.
If in situ denitrification is occurring, then it is reasonable to assume that it could be easily enhanced through the
introduction of a carbon substrate. A study was initiated to determine and demonstrate the feasibility of
enhancing denitrification in the local unconfined aquifers. The project consisted of two parts; a laboratory study
to determine the influence of various carbon substrates as well as the influence of flow rates and a field study. In
order to carry out a field demonstration, it was necessary to first determine that denitrification was occurring and
at what location within the aquifer. Three wells were installed for this purpose as part of a larger program
sponsored by the Geological Survey of Canada. One of these wells was located in the Abbotsford Aquifer
adjacent to the three Ministry wells. The other two were in the Brookswood Aquifer downgradient from an
Environment Canada demonstration site. It is the monitoring results from these three wells that is the focus of
this presentation.
The wells were monitored using a string of dialysis membranes which allowed for discrete sampling every 30
cm. The well in the Abbotsford Aquifer intercepted water that had passed under an extensive area of raspberry
canes. The Brookswood wells were located downgradient of residential properties serviced with septic tanks
(south well) and within a potato field (north well) but a short distance downgradient of treed property.
Conclusions
The water from the Abbotsford well contained nitrate levels in excess of 40 mg/L as N with a vertical gradient
and a pronounced seasonal variation in the zone influenced by the fluctuating water table. Variation was also
seen below the recharge zone but becoming less apparent with depth. The data did not support the likelihood of
significant in situ denitrification so an alternate mechanism for the nitrate gradients is suggested.
The analysis from the Brookswood wells showed a much lower concentration of nitrate in those groundwaters as
well as a limited depth of nitrate contaminate penetration. The south well, influenced by the housing, showed
little evidence of nitrate. The north well, approximately 300 m from the south well and obliquely downgradient,
appeared to be influenced by farming practices with nitrate contamination in the upper couple of metres.
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