On a good day, Jim Sandifer has $20 million to $30 million to drop into the money market. When he calls, brokers snap to attention.

But while Sandifer's investment portfolio is strictly blue-chip -- crammed with notes and bonds from financial giants like Chase Manhattan Corp. and the U.S. Treasury -- none of the money he invests in his. It belongs to the taxpayer of Fairfax County.

Sandifer, 35, is an assistant to the county finance director and in charge of investiments and banking. He is, in effect, the county's stockbroker.

Millions of dollars in revenue flow into the county's hands each year in the form of taxes, license fees, federal grants and bond monies, but it isn't all spent at one time. On any given day, the county has millions stockpiled but not yet spent.

Sandifer's job -- one that keeps him combing The Wall Street Journal and constantly on the phone to brokers in New York, Chicago, Miami and other major cities -- is to invest it, sometimes for as short a period as overnight.

Such short-term investments, earning more than if the money were left in the bank, are a lucrative source of additional revenues for local governments.

Fairfax, according to Sandifer, is expected to reap a profit of nearly $27 million in interest payments this year. Last year, it made $18 million, and the year before $16 million. The total county budget for the 1980 fiscal year is $640 million.

Other local governments in the region also reported sizeable interest incomes from investments.

Last year, Arlington made about $4.3 million, Alexandria, $1.1 million, Prince George's County, $8 million, Montgomery County, $11.6 million, and the District of Columbia, $10.3 million.

James P. McDonald, Fairfax's deputy county executive for management and budget, said he considers the county's investment program one of the best in the metropolitan area and in Virginia. "Very few counties have programs such as we have," he said.

In 1973, according to McDonald, the county hired financial consultants to analyze its cash flow and suggest ways to squeeze more investment money out of the county coffers.

The consultants found, for example, that only about 60 percent of county employes cash their paychecks on payday, which is Friday, and many checks don't clear the bank until the middle of the next week.

As a result, the county stopped covering all payroll checks on payday. Instead, McDonald said, it deposits enough funds to cover 60 per cent of the payroll.

It keeps the remainder, called the "float," in interest-bearing accounts over the weekend and then transfers it into the payroll account at the beginning of the next week.

"Money doesn't lie dead," McDonald said. "Somebody's always investing it. If you put it in a checking account, the bank's investing it. If you've got it, you're investing it."

In the case of taxes and other revenue checks sent to the county, the money is invested before county officials even get their hands on it.

Bank officials pick up the checks first at a post office box, credit them to the county's account immediately and then ship them to the county officials for processing.

"We do the administrative work after the money is deposited, rather than before," McDonald said.

And once the country has the money, it's up to Sandifer, who is paid about $30,000 a year to decide where to invest it.

The Board of Supervisors are kept abreast of the investments, but the county charter permits the director of finance and his assistants to invest county money without board approval, Sandifer said.

In a world of constantly changing interest rates, Sandifer said, "we can't wait until next week for board approval." That might be too late.

Sandifer said the county's investment guidelines require him to purchase mostly government or government approved obligations or certificates of deposit from Virginia banks. The county is barred by law from investing in common stocks.

Recently the state gave it the authority to buy short term, discount notes, called commercial paper, issued by large corporations. The county restricts its purchases to corporations that have a net worth of at least $50 million, net incomes of $3 million and that have the highest financial ratings.

Although the county makes conservative investments, Sandifer said, it pays almost no attention to how the borrowers use its money.

Sandifer said the main concern is getting a good rate of return.

The county, for example, owns $2 million in Israel Government National Securities, an investment that could be considered controversial.

Sandifer said the Israeli notes were guaranteed by the U.S. government and issued under its auspices as part of an effort to help that country. "If the federal government guarantees a bond, what difference does it make what is is?" he said.

Unlike county operating funds, the money in the county's four separate retirement plans can be used to buy common stocks. The retirement funds come from nearly equal contributions made by the county and its individual employes:

The Board of Superivsors and county officials have no direct say, however, on how the retirement money is invested.

And the retirement boards, Sandifer said, also do not look into the business practices of the companies they invest in.

An ordiance adopted in September 1977 by the Board of Supervisors banning smoking in certain public places apparently had no influence on the retirement boards' investment polcies. Each of the retirement boards has invested substantal sums in cigarette manufactures.

According to a 1978 county audit, the retirement fund for regular county employes invested $323,000 to purchase 6,000 shares of stock in R. j. Reynolds Industires, Inc. and $186,000 to buy 3,000 shares of Philip Morris.

"You have to realize," said County finance director Warren S. Hutchison, "these cigarette companies are involved in a lot of endeavors (besides making cigarettes)." Reynolds also manufactres food and alumum products. "It's pretty much up to the (outside) investment managers to make the first decision on what stocks to buy."