Detailed Instructions: This task is about writing a correct answer for the reading comprehension task. Based on the information provided in a given passage, you should identify the shortest continuous text span from the passage that serves as an answer to the given question. Avoid answers that are incorrect or provides incomplete justification for the question.
Problem:Passage: Many older buildings in certain areas of Boston are supported by wooden piles driven into the area's fill; these piles remain sound if submerged in water, but are subject to dry rot if exposed to air for long periods. Groundwater levels have been dropping, to varying degrees, in many areas of the city, due in part to an increase in the amount of rainwater discharged directly into sewers rather than absorbed by the ground. A city agency, the Boston Groundwater Trust, coordinates monitoring of groundwater levels throughout the city via a network of public and private monitoring wells. However, Boston's drinking water supply, from the Quabbin and Wachusett Reservoirs to the west, is one of the very few in the country so pure as to satisfy federal water quality standards without filtration. Question: What has been happening to groundwater levels?
Solution:
dropping