Fallen leaves and water quality
adapted from u. s. geological survey (usgs)

removing leaf litter does a lot more than make a yard look nice. the leaves can actually be harmful if left alone. a study in madison, wisconsin shows why.
leaf litter adds a large amount of phosphorus to urban storm water, which then runs off into waterways and lakes. excessive amounts of nutrients like phosphorus and nitrogen can be fatal for fish and other aquatic life. in madison, wisconsin, scientists tested exactly how much of the nutrients existed. without removal, leaf litter and other organic debris in the fall contributed 56 percent of the annual total phosphorus in urban storm water. this compares to only 16 percent when streets were cleared of leaves prior to rain.
this information proved to be very . "our study found that leaf removal is one of the few treatment options available to environmental managers for reducing the amount of dissolved nutrients in storm water," said bill selbig, a usgs scientist and the author of the report. "these findings are applicable to any city that is required to reduce phosphorus loads from urban areas."
from 2013 to 2015, scientists looked at concentrations of phosphorus and nitrogen in storm water from two areas in western madison that had similar tree cover. the city applied a leaf litter removal program from late september through mid-november at one site but not the other. the researchers found significantly lower amounts of phosphorus and nitrogen at the site where leaves were removed.
the study also found that storm water nutrient levels were highest during the fall. this finding shows that leaf removal programs are most effective during fall in madison. it also shows that sources other than leaves, such as street dirt and grass clippings, were likely the main contributors of phosphorus and other nutrients during spring and summer.
during the study period, the city of madison used leaf collection, street cleaning, and leaf blowers to remove leaf litter from residential areas. they also asked residents to pile their leaves next to the street to limit excess debris. leaf collection and street cleaning occurred about every seven days from late september through mid-november.
the study in madison has shed light on growing concerns for many environmentalists. the good news is that with proper maintenance, all cities can produce cleaner water.
the usgs collaborated with the city of madison, the fund for lake michigan, and the wisconsin department of natural resources on the study.

4
select all the correct answers.
which two statements explain how paragraph 4 contributes to the development of ideas in the passage?
a. it presents the idea that leaf removal processes need to be improved.
b. it introduces the importance of timing when removing leaf litter.
c. it highlights the effectiveness of the leaf removal program.
d. it shows that leaf litter can affect storm water in multiple locations.
c. it gives examples of the ways in which leaf litter is removed during fall.