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                                 EXHIBIT 10.13

                              FOURTH AMENDMENT TO
                   REVOLVING CREDIT AND TERM LOAN AGREEMENT


        THIS FOURTH AMENDMENT TO REVOLVING CREDIT AND TERM LOAN AGREEMENT
("Amendment"), is made and entered into effective as of February 15, 2000, by
and between NATIONAL ENVIRONMENTAL SERVICE CO., an Oklahoma corporation
("Borrower") and BANK OF OKLAHOMA, NATIONAL ASSOCIATION, a national banking
association ("Bank").

                                   RECITALS

        A. Reference is made to the Revolving Credit Loan Agreement dated
effective July 14, 1997, Amendment One to Revolving Credit Loan Agreement dated
December 2, 1997, Amendment Two to Revolving Credit Loan Agreement dated July 2,
1998, and Amendment Three to Revolving Credit Loan Agreement dated April 30,
1998, between Borrower and Bank (as amended, the "Credit Agreement"), pursuant
to which currently exists a $7,000,000 Revolving Line and a $4,000,000 Term
Loan. All terms used herein shall have the meanings given in the Credit
Agreement, unless otherwise expressly defined.

        B. Borrower has requested Bank to increase the principal amount of the
$7,000,000 Revolving Line to $9,000,000. Bank has agreed to accommodate
Borrower's request, subject to the terms and conditions set forth below.

                                   AGREEMENT

        For valuable consideration received, the parties agree to the following:

1.             AMENDMENTS TO CREDIT AGREEMENT.  The Credit Agreement is
               ------------------------------
amended as follows:

        1.1.        Section 1.53 is hereby amended to delete all references to
        the $7,000,000 Revolving Credit Note and replace with the "$9,000,000
        Revolving Credit Note". The form of the $9,000,000 Revolving Credit Note
        is attached as Schedule "1.4" to this Amendment. All references in the
                       -------------
        Credit Agreement to "$7,000,000" are hereby deleted and replaced with
        "$9,000,000".

2.        CONDITIONS PRECEDENT.  The making of any loan provided for herein,
          --------------------
unless otherwise noted herein or waived in writing by Bank, shall be conditioned
upon the Borrower executing and/or delivering the following:

        2.1.        This Amendment, with all schedules attached.

        2.2.        $9,000,000 Revolving Credit Note.
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        2.3.        Any other instruments, documents and/or agreements
        reasonably required by Bank in connection herewith, including without
        limitation any amendments to the real estate collateral documents
        together with applicable title assurances satisfactory to Bank.

3.             RATIFICATION. Borrower hereby ratifies and confirms all Loan
               ------------
Documents to which it is a party, and represents and warrants that: (i) the Loan
Documents remain in full force and effect and unchanged except as expressly
amended hereby, (ii) all representations and warranties made thereunder are true
and correct as of the date hereof and (iii) no Event of Default exists as of the
execution of this Amendment. Borrower further represents and warrants that the
Security Agreement and Deed of Trust delivered to Bank in connection with the
Credit Agreement remain in full force and effect, and that each shall
additionally secure payment of the $9,000,000 Revolving Credit Note and the
$4,000,000 Term Note, together with extensions, renewals and changes in form
thereof.

4.             NAME REPRESENTATION.  Borrower hereby represents and warrants to
               --------------------
Bank that its name has been changed to "NESCO, INC." and that evidence thereof
(e.g., copies of filed amendments with the appropriate government offices)
promptly shall be delivered to Bank; whereupon, the parties shall execute
appropriate UCC-1 amendments for filing.

5.             GOVERNING LAW AND BINDING EFFECT. This instrument shall be
               --------------------------------
governed by and construed in accordance with and governed by the laws of the
State of Oklahoma, and shall be binding upon and inure to the benefit of the
parties hereto, their respective heirs, executors, administrators, trustees,
successors and assigns.

6.             COSTS, EXPENSES AND FEES.  Borrower agrees to pay all costs,
               ------------------------
expenses and fees incurred in connection herewith.

                                 "Borrower"

                                 NATIONAL ENVIRONMENTAL SERVICE
                                 CO., an Oklahoma corporation

                                 By ________________________________________
                                 Larry G. Johnson - Vice President & Sec.-Treas.

                                 "Bank"

                                 BANK OF OKLAHOMA, NATIONAL ASSOCIATION

                                 By ________________________________________
                                   Kevin M. Lackner, Senior Vice President
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                                Schedule "1.4"
                                -------------
                      ($9,000,000 Revolving Credit Note)


                                PROMISSORY NOTE

$9,000,000                                                   February 15, 2000
                                                                 Tulsa, Oklahoma

         FOR VALUE RECEIVED, the undersigned, NATIONAL ENVIRONMENTAL SERVICE
COMPANY ("Maker"), promises to pay to the order of BANK OF OKLAHOMA, N.A.
("Lender"), at its offices in Tulsa, Oklahoma, the principal sum of NINE MILLION
AND NO/100THS DOLLARS ($9,000,000) or, if less, the aggregate sum of advances
made by Lender to Maker under the Revolving Credit and Term Loan Agreement
between Maker and Lender dated July 14, 1997, as amended, payable as follows:

         a.    Principal.  Principal shall be payable on April 30, 2002.
               ---------

         b.    Interest.  Interest shall be payable on the first day of each
               --------
               month, commencing the first day of March, 2000, and at maturity.
               Interest shall accrue on the principal balance outstanding
               hereunder and on any past due interest hereunder at a rate at all
               times equal to the Prime Rate (defined below) per annum.

If any payment shall be due on a Saturday or Sunday or upon any other day on
which state or national banks in the State of Oklahoma are closed for business
by virtue of a legal holiday for such banks, such payment shall be due and
payable on the next succeeding banking day and interest shall accrue to such
day. All interest due hereon shall be computed on the actual number of days
elapsed (365 or 366) based upon a 360-day year.

         Such installment payments are to be applied first to the payment of
interest on the principal balance from time to time remaining unpaid at the
aforesaid rate, and any balance shall be used to reduce the principal balance;
except that if any advances made by the holder hereof under the terms of any
instrument, document or agreement executed by Maker in connection herewith have
not been repaid, any monies received may, at the option of holder, be applied
first to repay such advances and interest thereon, and the balance, if any,
applied to any installment then due. Any prepayments shall be applied to
installments in the inverse order of occurrence.

         "Prime Rate" shall mean a fluctuating interest rate per annum as in
effect from time to time, which interest rate per annum shall at all times be
equal to the rate of interest announced publicly from time to time (whether or
not charged in each instance), by The Chase Manhattan Bank, N.A., at New York,
New York ("Rate Bank"), as its base rate or general reference rate. Each change
in the Prime Rate (or any component thereof) shall become effective hereunder
without notice to Maker (which notice is hereby expressly waived by Maker), on
the effective date of each such change. Should the Rate Bank abolish or abandon
the practice of announcing or publishing a Prime Rate, then the Prime Rate used
during the remaining term of this Note shall be that interest rate or other
general reference rate then in effect at the Rate Bank which, from

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time to time, in the reasonable judgment of Bank, most effectively approximates
the initial definition of the "Prime Rate." Maker acknowledges that Lender may,
from time to time, extend credit to other borrowers at rates of interest varying
from, and having no relationship to, the Prime Rate. The rate of interest
payable upon the indebtedness evidenced by this Note shall not, however, at any
time exceed the maximum rate of interest permitted under the laws of the State
of Oklahoma for loans of the type and character evidenced by this Note.

         All payments under this Note shall be made in legal tender of the
United States of America or in other immediately available funds at Lender's
office described above, and no credit shall be given for any payment received by
check, draft or other instrument or item until such time as the holder hereof
shall have received credit therefor from the holder's collecting agent or, in
the event no collecting agent is used, from the bank or other financial
institution upon which said check, draft or other instrument or item is drawn.

         From time to time the maturity date of this Note may be extended or
this Note may be renewed, in whole or in part, or a new note of different form
may be substituted for this Note and/or the rate of interest may be changed, or
changes may be made in consideration of loan extensions, and the holder, from
time to time, may waive or surrender, either in whole or in part, any rights,
guarantees, security interests or liens given for the benefit of the holder in
connection herewith; but no such occurrences shall in any manner affect, limit,
modify or otherwise impair any rights, guarantees or security of the holder not
specifically waived, released or surrendered in writing, nor shall any maker,
guarantor, endorser or any person who is or might be liable hereon, either
primarily or contingently, be released from such liability by reason of the
occurrence of any such event. The holder hereof, from time to time, shall have
the unlimited right to release any person who might be liable hereon; and such
release shall not affect or discharge the liability of any other person who is
or might be liable hereon.

         If any payment required by this Note to be made is not made when due,
or if any default occurs under any loan agreement or under the provisions of any
mortgage, security agreement, assignment, pledge or other document or agreement
which provides security for the indebtedness evidenced by this Note, the holder
hereof may, at its option, without notice or demand, declare this Note in
default and all indebtedness due and owing hereunder immediately due and
payable. Interest from the date of default on such principal balance and on any
past due interest hereunder shall accrue at the rate of five percent (5%) per
annum above the nondefault interest rate accruing hereunder. The Maker and any
endorsers, guarantors and sureties hereby severally waive protest, presentment,
demand, and notice of protest and nonpayment in case this Note or any payment
due hereunder is not paid when due; and they agree to any renewal, extension,
acceleration, postponement of the time of payment, substitution, exchange or
release of collateral and to the release of any party or person primarily or
contingently liable without prejudice to the holder and without notice to the
Maker or any endorser, guarantor or surety. Maker and any guarantor, endorser,
surety or any other person who is or may become liable hereon will, on demand,
pay all costs of collection, including reasonable attorney fees of the holder
hereof in attempting to enforce payment of this Note and reasonable attorney
fees for defending the validity of any document securing this Note as a valid
first and prior lien.

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         Upon the occurrence of any default hereunder, Lender shall have the
right, immediately and without further action by it, to set off against this
Note all money owed by Lender in any capacity to the Maker or any guarantor,
endorser or other person who is or might be liable for payment hereof, whether
or not due, and also to set off against all other liabilities of Maker to Lender
all money owed by Lender in any capacity to Maker; and Lender shall be deemed to
have exercised such right of setoff and to have made a charge against such money
immediately upon the occurrence of such default even though such charge is made
or entered into the books of Lender subsequently thereto.

         The holder of this Note may collect a late charge not to exceed an
amount equal to five percent (5%) of the amount of any payment which is not paid
within ten (10) days from the due date thereof, for the purposes of covering the
extra expenses involved in handling delinquent payments. This late charge
provision shall not be applicable in the event the holder hereof, at its option,
elects to receive interest at the increased rate as provided hereunder in the
event of default.

         This Note is given for an actual loan of money for business purposes
and not for personal, agricultural or residential purposes, and is executed and
delivered in the State of Oklahoma and shall be governed by and construed in
accordance with the laws of the State of Oklahoma.

                                         NATIONAL ENVIRONMENTAL
                                         SERVICE COMPANY


                                         By___________________________________
                                         Larry G. Johnson, Vice President &
                                                Secretary-Treasurer

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