What factor can negatively impact groundwater when mound systems are improperly designed?

Excessive layers of silt and clay in the design of mound systems can create significant issues for groundwater quality. These types of soil layers can inhibit water movement and drainage, leading to poor absorption of wastewater. When water doesn't infiltrate properly, it can accumulate at the surface and create conditions that promote runoff. This runoff may carry pollutants and nutrients directly into the groundwater or surface water bodies, adversely affecting overall water quality. Proper design seeks to ensure that the soils above and below the mound allow for adequate drainage, filtration, and absorption, thus protecting the surrounding environment and groundwater resources.

The other options may also present issues but do not directly influence groundwater quality in the same manner as excessive silt and clay layers. For instance, too little fill might lead to insufficient wastewater treatment but does not necessarily imply immediate contamination risk for groundwater. Proper vegetation cover is generally a protective measure for erosion and nutrient uptake, while incorrect slope can affect drainage but does not inherently result from soil type issues.