Today I will describe the groundwater/surface water interaction in the CRB in Mosier basin. 1
Ken Lite, studying groundwater resource in Mosier basin since 1980 s (photo by A. Bouchier) 2012 with Jonathan LaMarche studying GW/SW interaction. 2
The Columbia River Basalt Group consist of a sequence of Miocene flood basalts which were emplaced between 17.5 and 6 mya. The basalts cover 210,000 km 2 of northern Oregon, eastern Washington, and western Idaho. 3
Productive water-bearing zones are generally limited to thin breccias or vesicular flow tops or bottoms. This produces a stack of thin aquifers separated by thick, low-permeability basalt flow interiors. They are capable of very high well yields but are very susceptible to overdraft (low storage capacity, low vertical permeability). 4
Photo of basalt showing pillow-palagonite near The Dalles, OR (photo by A. Bouchier). 5
Like any other strata, streams can erode through the rock exposing multiple layers. 6
Mosier Creek has exposed the permeable zone of the Pomona basalt. 7
Pomona Member of the Saddle Mountain Basalts. Basalt Lolo of the Priest Rapids Member of the Wanapum Basalt. Basalt of Rosalia of the Priest Rapids Member of the Wanapum Basalt. Frenchman Springs Member of the Wanapum Basalt. Grande Ronde Basalt (recent evidence indicating the Grande Ronde Basalt does host an aquifer in the Mosier Basin). (Photo by A. Bouchier, hand samples collected by A. Bouchier) 8
WASC 2767 producing from Lolo. WASC 2760 producing from Pomona. 9
Hydrograph of water-level measurements for selected wells seasonal high and seasonal low. Newcomb (1963) reported static water-levels at 500 feet above sea level south of the thrust fault. 10
Couple of wells (WASC 2767 and WASC 52293) are completed into the Priest Rapids Lolo adjacent to stream do not appear to be declining. Well completed in the Pomona (WASC 2760) had a commingling well located nearby which influenced water levels in the Pomona well. Since ~2011 water level stable (commingling well was successfully abandoned in 2013). Wells completed in the Priest Rapids Rosalia and/or Frenchman Springs declined by ~ 1.1 meters per year from 1974 through ~2004. Rate of decline decreased to ~ 0.3 m/y until ~2014. Decline rate from 2014 through 2017 increased to ~1 m/y. Priest Rapids Lolo adjacent to stream are coincident or slightly above elevation of river. 11
Couple of wells (WASC 2767 and WASC 52293) are completed into the Priest Rapids Lolo adjacent to stream do not appear to be declining. Well completed in the Pomona (WASC 2760) had a commingling well located nearby which influenced water levels in the Pomona well. Since ~2011 water level stable (commingling well was successfully abandoned in 2013). Wells completed in the Priest Rapids Rosalia and/or Frenchman Springs declined by ~ 1.1 meters per year from 1974 through ~2004. Rate of decline decreased to ~ 0.3 m/y until ~2014. Decline rate from 2014 through 2017 increased to ~1 m/y. Priest Rapids Lolo adjacent to stream are coincident or slightly above elevation of river. 12
Contact point in Mosier Creek is at an elevation of ~330 ft asl. USGS stream gage just downstream from where West Fork of Mosier Creek joins the main branch. In 2012 OWRD installed a stream gage just over 1-mile downstream from USGS gaging site below the Pomona / Lolo contact point in the stream. 13
UGS gaging site just upstream of WASC 50811 OWRD gaging site here where property owner allowed OWRD to install 2 observation wells in 2015. 14
The gains/losses seen over this stretch are small relative to flow. Therefore, any flow above ~2 cfs has too large an error to statistically identify gains/losses. Can only tell during base-flow portion of season starting around mid-june. In the autumn the leaves fall, pile up in river and create ponding effect, which again throws off ability to statistically identify gains/losses starting around mid-september. In general we see this as a gaining reach early in the base-flow portion of the year, switching to a loosing reach around mid-july, then switching back to gaining reach around mid-august. Amount of gain (or loss) at the starting point were we can statistically identify gains/losses depends heavily on how wet a winter was just experienced. I want to point out that 2015 and 2016 are showing a different trend where the reach begins the base-flow season as a loosing reach but switch towards neutral or gaining at roughly the same time as other years. 15
2 observation wells drilled mid-2015. Instrumented with pressure transducers. Blue background areas indicate permitted irrigation season not that orchardists are irrigating the entire permitted season. Most of the pumping is from wells completed in the Priest Rapids and/or Frenchman Springs. Even our Lolo obs well, which is completed solely in the Lolo appears influenced by irrigation pumping. Pomona well and stream appear highly correlated. Lolo well and precipitation show correlation, supporting the idea that rainfall is recharging the basalt aquifers (in this case the Lolo) in uplands near the Columbia Hills anticline, as suggested by Newcomb (1963). 16
In mid-2016 installed low conductivity loggers in observation wells and stream. Stream sc makes sense heavy rainfall results in high discharge and lowers conductance of stream. Do not observe same pattern in Pomona well, even though do see head in this well respond to stream flow. Also do not see in Lolo well, not unexpected. Do want to draw your attention to these three dips in Pomona sc, will zoom in on this area. 17
Do want to draw your attention to these three dips in Pomona sc, will zoom in on this area. Originally used small parastaltic pump to purge wells and measure conductivity w/ handheld instruments. Inducing stream depletion from Mosier Creek! 18
Newcomb suggested that Mosier Creek gains water from the Priest Rapids Member where it is exposed along the creek bed (Newcomb, 1969). Ken suggested that, by the time of their investigation, the potentiometric surface of the Priest Rapids Member aquifer was below the surface elevation of Mosier Creek at the upper reaches (northwest corner of Section 19) (Lite and Grondin, 1988). We postulate that around 2014/2015, the potentiometric surface of the Priest Rapids Member aquifer declined below the surface elevation of Mosier Creek along the entire reach exposed by the creek and that it now acts as a sink for the Priest Rapids (and Frenchman Springs) Member aquifer(s). 19
(Photo by A. Bouchier) 20