Guest Column: Groundwater Concerns

Groundwater is often thought of as an invisible resource — out of sight out of mind. However, when the well runs dry, or conversely the water table rises to the surface of the land, groundwater comes to the forefront.

Residents and facilities east and northeast of Pablo were starkly reminded of this, starting in early August and continuing up to the present, when shallow groundwater started to emerge, flowing over roadways and flooding basements and low-lying areas.

We usually associate flooding with streams and rivers overtopping their banks, not high water table conditions or saturated soils flooding low-lying topography. What may be most perplexing in the Pablo area is the lag time between spring snowmelt and high stream flows, and the onset of high water table conditions.

Groundwater aquifers (subsurface reservoirs) around Pablo may be thought of as a stacked system, with a deep aquifer between 350 and 500 feet and a shallow perched aquifer that trends from the land surface south of Pablo to progressively greater depths northeast of Pablo and up to the Mission Mountains. Locally, we refer to these as the deep and the shallow Mud Creek aquifers.

Flooding is occurring in the shallow Mud Creek aquifer, and is predominantly related to well above average precipitation the area experienced starting in fall 2010 and continuing up until recently. Above average precipitation is partly attributed to a La Nina ocean circulation pattern, and this pattern is forecast to persist into winter 2012 http://www.elnino.noaa.gov/forecast.html.

What does that mean locally, here in the Mission Valley? We had well above average fall 2010 precipitation that, by the late spring and summer of 2011, accumulated a snowpack in the Mission Mountains that was 150 to 180 percent of the average. Well below average June and July temperatures delayed snowmelt, allowing a larger amount of water to infiltrate into the ground, recharging aquifers and soil profiles. Groundwater recharge moves slowly through subsurface sediments from mountainous areas to valley areas, on the order of feet per day, and emerged in the Pablo area in early August.

What do we know and what is the outlook for this fall? We know that there are large areas of mapped wetlands and peat soils northeast of Pablo. These tell us that very shallow water table conditions have occurred here for many decades, and more likely much longer than that. Looking forward, we know that current flows in nearby streams are 150 to 200 percent of average http://waterdata.usgs.gov/mt/nwis/rt and are supported by groundwater at this time of year.

This suggests that groundwater fluxes continue to be high and, combined with the potential for ongoing La Nina conditions, suggest that high water table conditions may continue into the fall in areas affected by groundwater flooding.

Resources are available to learn more about flood issues.

A good starting point is the following website http://www.deq.mt.gov/wqinfo/pws/security/floods.mcpx.

You can also contact a Lake County or tribal representative to further address your concerns (Lake County Environmental Health Department at 883–7236 or Tribal DES Program at 675-2700 x6414).