Final answer:
The question pertains to the safe distance required between excavation activities and the seasonal groundwater level to prevent contamination. The addition of water to a recharge area increases groundwater, and maintaining the pre-development level of the water table is critical for ecosystem health.
Step-by-step explanation:
The question deals with the standards for the safe distance of excavation activities from the highest known seasonal groundwater level to prevent contamination of groundwater or surface water. While the exact answer to the blank in the question cannot be provided without referencing specific regulations or guidelines that vary by location, the general concept is to maintain a safe vertical distance to prevent contamination from gray water used in irrigation fields or disposal.
Regarding the second part of the question, the addition of water to a recharge area typically leads to an increased amount of groundwater, which would be the correct answer for a practical effect of this action. Contrarily, the removal of groundwater can result in a lowered water table or the creation of a "cone of depression" which could affect nearby wells and water bodies.
Finally, it's crucial to maintain the pre-development level of the water table because it supports the ecosystem's biodiversity and helps contain the spread of pollutants by preventing their widespread dissemination through groundwater flow.