Chapter 5 – Groundwater Levels

Release Date: December 2012

Groundwater is a vital natural resource in the Credit River Watershed. Groundwater levels are an indicator of the amount of groundwater in aquifers and shallower water tables.

Groundwater contributions to streams, rivers and wetlands play a crucial role in maintaining surface water quantity, quality and temperature, all vital to support the health of aquatic ecosystems. Groundwater is also an important source of drinking water for approximately 116,000 people living in the Credit River Watershed (CTC 2012).

Groundwater originates as surface water that percolates through layers of soil and rock and is stored beneath the earth’s surface in aquifers. Aquifers in the Credit River Watershed are typically comprised of either older deep-seated bedrock material such as dolostone, sandstone and limestone, or of more recent overburden material such as gravel, sand and silt (AquaResource Inc. 2009). Groundwater level is the depth below the earth’s surface that is saturated with water, or the level to which groundwater would rise in a well that is drilled in a confined (pressurized) aquifer (Figure 1).

Figure 1: Monitoring groundwater at the Robert Baker well

Typically, groundwater moves from areas of high hydraulic head to areas of low hydraulic head. When a stream crosses the water table, groundwater may enter the stream and contribute to flow. Approximately half of the Credit River’s average flow comes from groundwater and during periods of drought, groundwater (i.e. baseflows) becomes the primary source of water to streams (AquaResource 2009). This baseflow is an important component of streamflow in the Credit River and is critical for maintaining the structure and function of the Credit River ecosystem.

Groundwater Level Monitoring

CVC monitors groundwater level in 14 wells at 9 locations throughout the Credit River Watershed (Figure 2). These monitoring wells are located in important groundwater recharge areas. They were established in 2001 through a partnership with the Ontario Ministry of the Environment’s Provincial Groundwater Monitoring Network (PGMN).

Five of the monitoring locations are comprised of nested wells (i.e. a coupled shallow and deep well) allowing for groundwater conditions to be compared in different aquifers at the same surface location.

Groundwater levels at each well are indirectly measured with a piece of equipment known as a pressure transducer water level logger. This logger records and digitally stores groundwater level data at hourly intervals. CVC also collects manual groundwater level measurements monthly to verify the values recorded by the pressure transducer.

Groundwater levels are recorded as meters above sea level (masl). When comparisons among wells are made, groundwater levels are expressed as the relative change in water level at each well in meters (m). Average seasonal and annual groundwater levels are calculated for each well. If, however, more than 10 percent of the record is missing, (i.e. an annual average missing more than 36 days) averages from those seasons or years are omitted from long-term trend analysis.

Groundwater Level Status (2011)

Over all monitoring sites, groundwater levels for most wells in 2011 were consistent with long-term records. Typical seasonality in the groundwater level is evident throughout 2011 with water levels increasing during spring melt and reaching their lowest levels in autumn and early winter (Figure 3). The magnitude of change in relative groundwater level in response to seasonal climatic fluctuations is heavily influenced by the depth of the aquifer and the porosity of material surrounding and within the aquifer. For example, the Erin well is relatively shallow and surrounded by porous material and has a greater response to seasonal fluctuations in precipitation. In contrast, the Caledon well is covered by a very thick unsaturated zone and shows little appreciable change in groundwater level in response to seasonal changes in precipitation. (Figure 3)

Figure 3: Change in average monthly relative groundwater level and total monthly precipitation for the Upper Watershed in 2011.

Groundwater Level Trends

The seasonal patterns of groundwater level evident in 2011 are part of the longer-term trend of fluctuating groundwater levels in a lagged response to shifts in precipitation (Figure 4). The time lag between shifts in precipitation and groundwater level can vary from minutes to centuries and is dependent on a number of factors including substrate type and depth of aquifer, soil saturation levels, land cover and the type, quantity and rate of precipitation. Therefore, in addition to human groundwater usage, climate change and urbanization can also have sizeable impacts on groundwater levels by altering precipitation patterns and increasing impervious surfaces which reduce the rate of groundwater recharge.

Figure 4: Erin well groundwater level (2002 – 2011)

Overall, there was little statistically significant change in average annual groundwater levels across the watershed over the 11-year monitoring period (See Table A1 for results of statistical trend analysis). Over half (9 out of 14) of the wells showed no statistically significant change in average annual groundwater levels over the monitoring period (2001-2011). It is important to note, however, that the groundwater monitoring records are still considered fairly short. As the length of monitoring records increase, so will the ability to detect statistically significant trends.

In the Upper Watershed, groundwater level showed no statistically significant change in four of the six wells (Orangeville Shallow, Erin, Hillsburgh Shallow and Hillsburgh Deep) and a significant increasing trend in the Caledon well. One monitoring well, the Orangeville Deep well, had a significant decline in groundwater level with a 1.4 m drop in average annual groundwater levels from 2005 to 2007 (Figure 5). During this period, the area surrounding this monitoring well underwent residential development and a number of private wells were completed in the same bedrock aquifer. This suggests local land use change may be responsible for observed groundwater level drawdown. Since 2007, however, the average annual groundwater level at this location has been increasing and in 2011 was 1 m lower than the 2005 level, indicating a partial recovery of groundwater levels at this location. CVC will continue to closely monitor groundwater levels in this well to assess if this recovery trend will continue.

Figure 5: Examples of significant trends in average annual groundwater level for three monitoring wells in the Upper and Middle Watershed

In the Middle Watershed, there was no significant trend in average annual groundwater levels at four of seven monitoring wells (Warwick Shallow, Robert Baker Shallow, Robert Baker Deep, and Acton). A statistically significant trend of increasing groundwater levels was observed in the Warwick Deep well, suggesting groundwater levels are increasing at this site (Figure 5). The increase in groundwater level at Warwick Deep may be a cumulative response to increasing annual precipitation over the monitoring period. Due to the depth of the well (~200 m) and the confined nature of the aquifer, movement in groundwater level typically exhibits a lagged response to variations in precipitation. Two wells (Georgetown Shallow and Georgetown Deep) showed a significant decline in groundwater levels (Georgetown Deep shown in Figure 5). These two wells are nested in close proximity to Georgetown municipal wells and the downward trend suggests a localized decrease in groundwater levels, likely resulting from increased local groundwater use.

The single monitoring well in the Lower Watershed (Huttonville) showed no statistically significant change in groundwater level.

Conclusions

In most wells the data collected to date suggests that land and water use are having minimal impact on groundwater quantity. There may, however, be more localized changes to groundwater levels driven by human activities. For example in the Georgetown and Orangeville wells average annual groundwater levels fell by 2.9 and 1.4 m respectively during the monitoring period, indicating a reduction in groundwater quantity at these locations. Although the Orangeville well is showing some signs of recovery in groundwater level, the Georgetown wells appear to be continuing to follow a downward trend indicating a pattern of reduced groundwater quantity at this location. CVC’s groundwater monitoring program was established in 2001 and as we continue to monitor groundwater, our ability to detect long-term trends in groundwater level will be strengthened. This will aid in making informed decisions for effective management of this important resource, which is critical to both ecosystem and human health in the Credit River Watershed.

Chapter 6 will continue on the topic of groundwater, focussing on groundwater quality. Groundwater is the single source of drinking water for thousands of people and is essential for the Credit River ecosystem. Is there evidence that human activities are altering the quality of our groundwater?

Did you know?

Groundwater is the source of drinking water for about 116,000 residents of the Credit River Watershed (CTC 2012).

 

References

AquaResource. 2009. SPC Accepted Integrated Water Budget Report – Tier 2, Credit Valley Source Protection Area, April 2009

CTC Source Protection Region. 2012. Proposed Source Protection Plan: CTC Source Protection Region.

Table A1: Results from linear regression analysis of average annual groundwater level versus monitoring year. Statistically significant results (p<0.05) are shown in bold.

Monitoring well

Sample size (n)

Coefficient of determination (r2)

Slope

p-value

Upper Watershed

Orangeville Deep

9

0.52

-0.11

0.03

Orangeville Shallow

6

0.03

0.01

0.75

Erin

7

0.724

0.07

0.10

Caledon

7

0.83

0.18

0.004

Hillsburgh Deep

5

0.55

-0.02

0.15

Hillsburgh Shallow

4

0.54

-0.02

0.26

Middle Watershed

Georgetown Deep

8

0.58

-0.20

0.03

Georgetown Shallow

9

0.49

-0.14

0.04

Warwick Deep

7

0.77

0.09

0.01

Warwick Shallow

5

0.61

0.06

0.12

Robert Baker Deep

8

0.12

0.04

0.40

Robert Baker Shallow

8

0.114

0.04

0.41

Acton

8

0.45

0.19

0.06

Lower Watershed

Huttonville

6

0.35

-0.10

0.22

 

 

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