This is Tom's Post

On Thursday, August 18, the technical team was scouting the banks of Rio Moche for a suitable location for water quality monitoring. Team members observed an area where surface vegetation and surface soils had been removed. Upon closer examination of this area they observed massive crack in the soil characteristic of those formed when soils with a high clay content are dried. They marked this site to return for infiltrometry measurements.

On Friday, August 19, the technical team did conduct infiltrometry measurements and found that this location required 55 minutes to absorb 15 milliliters of water from the water reservoir. This infiltration rate corresponds to clay soils according to the users manual. The device used was a Mini Disk infiltrometer manufactured by Decagon Devices with the suction control tube set at 2 cm.

On Saturday, August 20 the technical team conducted a second on another portion of the site and found 45 minutes required for drawdown of 15 milliliters which again indicates a tight soil but not as tight as the former site. For comparison purposes the team conducted an infiltration measurement on a bank of sand on the edge of the river and measured a drawdown of 80 milliliters in 30 seconds, characteristic of a coarse sand.

This area and the clay content are significant for the following reason. When a sanitary landfill is constructed in Samne there must be a barrier to prevent landfill leachate from percolating into ground water. Deposits of low permeability clay could potentially be used in lieu of or in addition to a synthetic membrane.

We subsequently learned that this disturbed area is controlled by a mining company that is prospecting for silver. There were workers in the area handling drums of chemicals and mixing them in shallow impoundments.

The outcrop of soil material is obviously sedimentary in nature as the vertical profile has distinct bands of perfectly horizontal strata which vary in color and texture. These horizontal deposits are on top of eroded rubble and boulders on the valley floor. The geomorphology of this site is unknown to the team but the following mechanism is postulated as an imaginary yet plausible sequence in order to further describe these deposits. Perhaps at some time in the remote past a landslide or avalanche blocked the flow of the river and created a large lake. Over a long period of time this lake became a sedimentation basin in which eroded material carried by the river settled under the influence of gravity, and following Stokes Law, the largest particles settled at the upper, shallow end of the lake and the finer materials settled in the deeper regions of the lake. These sediments would then overlay the existing rubble in the river bed as observed. Eventually the material blocking the river eroded away and the lake drained and the sediments emerged as dry ground.

If the preceding hypothesis is reasonably correct then it follows that this large deposit of fine-grained soil material may be a rare phenomenon in the Rio Moche valley. Elsewhere in the Samne region all exposed soil profiles are unconsolidated deposits comprised of rock and soil mixed randomly. This particular outcrop along the river is the only location observed where there is fine soil free of rock. This being the case we recommend that this outcrop be preserved for as source of materials for construction and operation of the landfill.

This exact site is not appropriate for the location of a landfill because it is within the valley bottom and the elevation is probably below the level of peak floods. Therefore a landfill at this site could be eroded during a flood and buried waste would be washed loose and into the river flow. For clarity, it is restated that we consider this material to be good for the construction of a landfill but this exact location to be unsuitable for a landfill.