Answer: Movement of pollutants through the groundwater.
Step-by-step explanation:
Groundwater is the water that accumulates below the surface of earth crust. This water forms the part of geosphere. The water from river, lakes, ponds and rain seep below the surface of ground through small pores present between the soil aggregates and rocks.
According to the given situation, sand and gravel forms the part of earth crust. The water here is the part of groundwater. The food coloring here is the example of pollutants which seep inside the pores present between the sand and gravel. Hence, polluting groundwater.