About 350 trees and several hundred azalea bushes have drowned at the 9-month-old Constitution Gardens, according to the National Park Service, apparently because water has had trouble seeping through the almost impenetrable clay and rubble upon which Washington's newest park was built.

The Park Service is now spending $140,000 to replace the azaleas and 140 large trees, most along and park's two miles of winding gravel paths, and expects to spend an additional $100,000 next year to replace even more.

Death by drowning was the verdict reached this winter following a horticultural autopsy on the trees and shrubs, conducted by Park Service tree specialists, and scientists from the Department of Agriculture and U.S. Geological Survey.

The 45-acre park and its six-acre lake along-side the Reflecting Pool were created from the bulldozed foundations of 18 "temporary" Navy and munitions buildings that squatted on the site from World War I until they were demolished in 1970. To this were added thousands of tons of heavy subsoil from other federal construction projects, all topped by 18 inches of leaf mulch, composted sewage and a little topsoil.

"The top 18 inches is fine but when we dug deeper to put in large trees and to put in trees along the paths, water collected in the holes like a tea cup," says Ed Peetz, the Park Service official who watched over the whirlwind finish of $6.7 million park. It was dedicated last May 27 for the Bicentennial, with workmen still installing sod the day before the ceremonial opening and spraying green paint on the freshy seeded mud.

"Among our problems were that the project was held up for two years, we had a trucking strike and a wet spring, which made it hard to work, and our budget was cut in half," says Peetz. Oringinally estimated cost $16 million to $17 million, the park budget was reduced to $12 million, then to $6.7 million. "It was . . . (Secretary of the Interior Rogers C.B.) Morton's contributions to the austerity program," Peetz said.

One of the items cut was large quantities of topsoil, which would have provided the park with good earth deeper than 18 inches. Instead, the Park Service used 9,000 tons of sewage sludge from the Blue Plains Sewage Treatment Plant that had been composted with wood chips, and huge quantities of composted leaves from fall leaft collections.

The head of a private reclamation firm in McLean told the Park Service he felt the trees have all died from an overdose of sludge, which he said the Park Service used only because it was free instead of using "clean fill" (topsoil).

"That's completely false," says Peetz "the composted sludge is absolutely not a fault. We use it extensively in all the parks . . . all over the place. The Department of Agriculture and the U.S. Geological Survey concluded that water did it. The trees drowned."

The Park Service conducted an extensive postmortem to insure the contractors weren't at fault, said Peetz, because large trees, many of them 8 inches in diameter, are expensive to replace. "We concluded it was nobody's fault. The trees were the victims of a combination of things."

The 350 dead trees, about half of all the large trees planted in the park and most of the trees planted actually in the park's gravel pathways, "still only represent about 10 per cent of all the trees we planted," says Peetz, and the equipment area was the last section of the park to be finished.

The tree boxes for the pathway trees were thus dug directly in the "solid, almost impermeable soil," says Peetz," and although good mulch was put in with them they all just filled up with water. You could shake the tree and hear it slosh. They were like bowls of soup." The fact that Park Service maintenance staff watered the park heavily all summer to help the new grass also helped keep the trees soggy, said Peetz.

Many of the trees on the bulldozed hill near 17th Street, where heavy construction equipment and trailers were kept until only weeks before the park opened, also found themselves in watery graves despite the topping of leaf and sludge compost. An effort was made to mix the mulch and sludge with the heavy soil but Peetz said this was not as successful as officials had hoped becuase of the rush to open the park and the heavy spring rains that made mixing difficult.

Even though conditions for the new trees will be improved with new grading, drainage and even pipes for sump pumps to keep trees from getting their feet too wet, the Park Service is also going to plant different, "more water-tolerant varieties," says Peetz, "maybe cyprus . . . well, no, but there are some hybrid maples and some oaks that like water. They'll have to."

Some Park Service officials wondered if the fact that much of the Mall area was once tidal marsh, including the Lincoln and Jefferson Memorials and Constitution Gardens, might be at the bottom of the trees' drowning. The area was filled in with river dredgings about the turn of the century and the Constitution Gardens site was used briefly as a park before the "temporary" three-story Navy and munitions buildings were constructed in 1917 on concrete pilings, because of the spony soil.

The 18 buildings were connected by almost 100 first, second and third-story was added in World War II, and they housed 15,000 government employees and thousands of tons of equipment. The load apparently became too great because some of the buildings began sinking and were evacuated in 1968.

Most of the park trees that drowned last summer and fall are on hills and paths well above surrounding parkland and Constitution Avenue - which in the last century was a canal extending from Georgetown to the foot of the Capitol - and apparently drowned in water from above, from rain and sprinklers, not from below, says Peetz.