Deposits at banks based in the District rose about 8 percent last year to a record $6.3 billion as of Dec. 31, according to a survey by The Washington Post.

However, the rate of growth was off considerably from the 17 percent expansion during the previous year and the 14 percent rate of growth during the 12 months ended last June 30.

The slowdown in bank deposits at the city's banks reflects not only a general slowing of the economy but also the impact of new investment vehicles during an era of record interest rates, as customers took money out of banks and placed their funds in high-paying money market certificates or money market mutual funds.

With the exception of three minority-owned institutions -- Diplomat National, Industrial and United National -- all D.C. banks reported increased deposits in 1979. Riggs National remained the largest area bank with deposits of $2.18 billion. D.C. BANKS (TABLE) (COLUMN)Dec. 31, 1979(COLUMN)Dec. 31, 1979(COLUMN)Dec. 31, 1978 Amer Indian Nat(COLUMN)$14,867,141(COLUMN)$13,202,235(COLUMN)$15,854,531 ASBank(COLUMN)1,800,000,000(COLUMN)1,662,000,000(COLUMN)1,291,671,952 Bk of Columbia(COLUMN)45,574,789(COLUMN)41,442,378(COLUMN)37,044,177 DC National(COLUMN)90,604,865(COLUMN)83,193,560(COLUMN)68,878,987 Diplomat Nat(COLUMN)7,248,000(COLUMN)8,063,649(COLUMN)9,679,826 Hemisphere Nat(COLUMN)21,015,536(COLUMN)11,550,651(COLUMN)9,579,678 Industrial Bk(COLUMN)50,276,666(COLUMN)50,882,870(COLUMN)49,740,039 Madison Nat(COLUMN)133,553,234(COLUMN)112,728,254(COLUMN)102,013,840 McLachlen(COLUMN)45,576,702(COLUMN)40,481,537(COLUMN)38,734,409 Nat Bk Wash(COLUMN)744,718,885(COLUMN)690,673,165(COLUMN)630,054,674 Nat Capital(COLUMN)39,608,860(COLUMN)34,200,717(COLUMN)35,278,760 NS&T(COLUMN)431,133,684(COLUMN)422,272,330(COLUMN)340,441,821 Riggs(COLUMN)2,183,306,993(COLUMN)2,032,025,460(COLUMN)1,741,733,479 Security Nat(COLUMN)95,324,269(COLUMN)89,194,960(COLUMN)79,671,367 Union First(COLUMN)539,614,865(COLUMN)500,889,355(COLUMN)463,418,889 United Nat(COLUMN)52,310,000(COLUMN)52,393,000(COLUMN)31,459,090 Women's Nat BK-a(COLUMN)8,338,998(COLUMN)4,655,000(COLUMN)- TOTALS(COLUMN)$6,303,073,487(COLUMN)$5,849,849,121(COLUMN)$4,945,255,519(END TABLE) a new bank