<PAGE>
 
Exhibit 10.18  

                               December 16, 1998

               Kenneth L. Hadnott, Charles D. Harrison, and Brian Gray; and THH
               Alpha, Inc.; THH Services, LTD.; UTTS Services, LTD.; UTTS, Inc.;
               Carolina Drilling Services, Inc.
               P. O. Box 8148
               Greenville, NC 27835-8148

               Re: Acquisition of certain assets of THH Alpha, Inc.; THH
               Services, LTD.; UTTS Services, LTD.; UTTS, Inc.; Carolina
               Drilling Services, Inc. (hereafter "Companies") from Kenneth L.
               Hadnott, Charles D. Harrison, and Brian Gray, (hereafter
               "Shareholders")

               Gentlemen:

                    The Definitive Agreement, dated December 4, 1998, contained
               the terms of the acquisition by National Environmental Service
               Company (hereafter "NESCO") of the assets and assumption of
               certain liabilities and obligations of the Companies. The terms
               and conditions of the Definitive Agreement dated December 4, 1998
               are hereby amended by the terms and conditions presented in this
               document. The effective date of the acquisition shall be January
               1, 1999. Notwithstanding anything contained herein to the
               contrary, however, the covenants and agreements of the parties
               expressed in paragraphs 10, 11, and 12 hereof shall be binding
               regardless of whether or not this Definitive Agreement is
               consummated.

                    Shareholders have a desire to sell certain assets and
               dispose of certain liabilities at its Greenville, North Carolina,
               offices, and NESCO proposes to purchase those assets and assume
               those liabilities under the following terms and conditions:

                    1.  Assets to be Purchased -
                        ----------------------  

                            A.  Accounts Receivable - All accounts receivable
               shall be acquired by NESCO except for those state indemnity fund
               receivables owned by UTTS Services, LTD. UTTS Services, LTD.
               shall retain the state indemnity fund receivables owned by it as
               of the date of closing. UTTS Services, LTD. shall collect those
               receivables and remit proceeds to NESCO without additional
               payment to Shareholders by NESCO. UTTS Services, LTD. shall
               execute a Note payable to NESCO at the date of closing in an
               amount equal to the accounts receivable retained by UTTS
               Services, LTD. Remittance to NESCO of all such accounts
               receivable amounts collected by UTTS Services, LTD. shall
               constitute payment of Note.
<PAGE>
 
               NESCO Definitive Agreement - December 16, 1998 - Page 2

                            B.  Work in Progress

                            C.  Fixed Assets - NESCO agrees to purchase for cash
               and/or notes all fixed assets as shown on the most recent
               financial statements of Companies and provided to NESCO by
               Shareholders.

                            D.  Business Property - (a) all designs, methods,
               inventions and know-how related thereto, (b) all trademarks,
               trade names (including "UTTS Environmental"), service marks, and
               copyrights claimed or used by Companies that have not been
               registered, and (c) all customer lists of Companies, and (d)
               corporate names used by the Companies (collectively "Business
               Property Rights"), constitute all such proprietary rights owned
               or held by Companies and which are reasonably necessary to, or
               used in the conduct of the business of Companies. All such
               methods, inventions and know-how constitute trade secrets of
               Companies within the meaning of all applicable laws, and
               Companies have taken all necessary steps required by law to
               protect these trade secrets as such. Companies own or have valid
               rights to use all Business Property Rights without conflict with
               the rights of others. No person has made or threatened to make
               any claims that the operation of the business of Companies is in
               violation of or infringes any Business Property Rights or any
               other proprietary or trade rights of any third person. No third
               person is in violation of or is infringing upon any Business
               Property Rights.

                            E.  Other Assets - All other assets shown on the
               most recent financial statements of Companies presented to NESCO
               including but not limited to cash, bank accounts, deposits, and
               prepayments.

                    2.  Assumption of Lease - NESCO agrees to assume the 
                        -------------------    
               liability for the remainder of the payments on Companies's
               Greenville office building lease. It is a condition of this
               agreement that NESCO be able to assume said lease and occupy the
               premises.

                    3.  Liabilities - It is understood that NESCO will assume 
                        -----------           
               all liabilities as shown on the December 31, 1998, financial
               statements but excluding any federal and state income taxes due
               and payable. NESCO shall not be responsible for any liabilities
               not shown on such statement. Said liabilities shall not exceed
               amounts shown on the most recent financial statements presented
               to NESCO by Companies.
<PAGE>
 
               NESCO Definitive Agreement - December 16, 1998 - Page 3

                    4.  Assignment - This agreement is contingent upon the 
                        ----------      
               Companies' ability to assign without penalty or loss to NESCO all
               existing contracts, backlog, customer lists, deposits, good and
               merchantable title to assets, accounts receivable, receivables
               accrued as work in progress and not invoiced, licenses, and such
               other rights as Companies presently enjoys at their Greenville
               office.

                    5.  Acquisition of THH Alpha, Inc. - It is understood that 
                        -----------------------------       
               the acquisition of all the issued and outstanding shares of THH
               Alpha, Inc. is a part of the acquisition covered by this
               Definitive Agreement. As soon as practical after the Closing Date
               of the Transaction, NESCO and Shareholders of THH Alpha, Inc.
               will take the necessary actions to transfer ownership of the
               shares of THH Alpha, Inc. to NESCO while preserving the status of
               the corporation as a professional geological corporation under
               the laws of the State of North Carolina.

                    6.  Purchase Price -
                        --------------  

                            A.  Cash - $500,000.00 payable as follows:

<TABLE> 
<CAPTION> 
                                                   One Year        Two Years
                                     At Closing    After Closing   After Closing
                                     ----------    -------------   -------------
               <S>                   <C>           <C>             <C> 
               Kenneth L. Hadnott    $100,000      $50,000         $50,000
               Charles D. Harrison   $100,000      $50,000         $50,000
               Brian Gray            $ 50,000      $25,000         $25,000
</TABLE> 

                            B.  NESCO Common Stock - Kenneth L. Hadnott and
               Charles D. Harrison shall each receive 20,000 shares of Rule 144
               NESCO common stock issued from the treasury shares presently
               owned by NESCO.
               
                            C.  Stock Options - Options to purchase NESCO Common
               Stock to be issued from the authorized but unissued shares at
               $2.00 per share in accordance with the National Environmental
               Service Company Stock Option Plan as follows:

<TABLE> 
                            <S>                         <C> 
                            Kenneth L. Hadnott          20,000 Shares
                            Charles D. Harrison         20,000 Shares
                            Brian Gray                   5,000 Shares
</TABLE> 
<PAGE>
 
               NESCO Definitive Agreement - December 16, 1998 - Page 4

                            D.  Deduction for Sellers' Income Tax Liability -
               Shareholders of THH Services, Inc. shall be permitted to deduct
               from THH Services, Inc.'s cash the amount of unpaid income taxes
               required on net income generated by THH Services, Inc. from the
               beginning of the current tax year through December 31, 1998. Any
               tax liabilities created by this transaction will be the
               responsibility of the Companies' Shareholders.

                            E.  Employment Contracts - Kenneth L. Hadnott,
               Charles D. Harrison, and Brian Gray shall each enter into a two-
               year employment contract with NESCO.

                    7.  Indemnification - Companies and Shareholders shall 
                        ---------------      
               indemnify NESCO for any claims on work performed by Companies or
               its subcontractors on or prior to December 31, 1998. Stockholders
               shall indemnify NESCO and hold it harmless for all claims for a
               period of five (5) years after the closing date which are not
               reflected on the December 31, 1998 Balance Sheet and disclosed by
               Shareholders and Companies and will pay all costs, expenses, and
               reasonable attorney's fees in respect to any tax and other
               accrued absolute or contingent liabilities or claims against
               Companies existing at the time of the Closing, or arising out of
               pre-closing transactions or events. However, Shareholders will
               not be liable for claims against Companies to the extent that the
               obligations the Companies are required to perform after the
               Closing Date in the ordinary course of business, or under
               agreements mentioned in or consistent with this transaction.
               Shareholders shall repurchase all accounts receivable (excluding
               accounts receivable due from state indemnity funds) purchased at
               Closing but which remain unpaid ninety (90) days after Closing
               date. Shareholders shall repurchase all accounts receivable due
               from state indemnity funds purchased at closing but which remain
               unpaid two hundred seventy (270) days after Closing Date.
               Shareholders shall indemnify NESCO against any and all claims
               from third parties made pursuant to any laws and/or regulations
               covering the bulk sale of inventory and assets.

                    8.  Contingency - Shareholders and Companies acknowledge 
                        -----------            
               that NESCO's decision to enter into the definitive agreement is
               particularly dependent upon (i) the satisfactory completion by
               NESCO of an investigation of the business and assets of
               Companies, (ii) arrangements satisfactory to NESCO concerning the
               continued employment following consummation of the Transaction of
               certain key employees of the Company, and (iii) approval of the
               Transaction by the Board of Directors of NESCO and Companies.
               This Definitive Agreement will be conditioned upon,
<PAGE>
 
               NESCO Definitive Agreement - December 16, 1998 - Page 5

               among other things, (i) approval of the Transaction by the
               requisite vote of the shareholders of Companies, (ii) all
               necessary governmental approvals having been obtained, and (iii)
               all necessary debt holder or other contractual consents having
               been obtained.

                    9.  Access to Personnel, Books and Records and Properties -
                        ----------------------------------------------------- 
               At all times prior to the consummation of the Transaction,
               Companies shall afford NESCO and its representatives, agents, and
               employees full access to the personnel, books and records,
               tangible assets, agreements, and licenses of Companies as may be
               reasonably requested by NESCO or its representatives, agents, or
               employees. Companies agree that prior to closing the Transaction,
               NESCO will be furnished with such accounting information and
               reports to the extent NESCO deems necessary to enable NESCO to
               perform its public reporting obligations and to satisfy the
               disclosure requirements under the rules and regulations of the
               Securities and Exchange Commission.

                    10.  Confidentiality - Each party shall keep confidential 
                         ---------------    
               any information with respect to the other party that is not
               otherwise generally available to the public or has been made
               available to the public by persons other than such party, its
               representatives, agents, or employees, except as is necessary in
               connection with the preparation of the definitive agreement
               relating to the transaction or as may be required by applicable
               law or stock exchange rules. If for any reason the transaction is
               abandoned or terminated prior to being consummated, each party
               will return promptly all information containing confidential or
               proprietary information disclosed to the other party.

                    11.  Publicity - Except as may be required of NESCO by 
                         ---------        
               applicable law or securities laws or rules, neither party hereto
               shall make any public announcement or any press release regarding
               this proposal or the transaction or the subject matter hereof or
               thereof without the prior consent of the other party.

                    12.  Expenses - Each party hereto will separately bear its 
                         --------        
               own expenses incurred in connection with this proposal and the
               transaction, regardless of whether the transaction is
               consummated.
               
                    13.  Interim Operations - Shareholders and Companies shall 
                         ------------------  
               continue to operate in a manner which will maintain the
               Companies' Greenville office's value, customers, reputation, and
               goodwill during the time period between the
<PAGE>
 
               NESCO Definitive Agreement - December 16, 1998 - Page 6

               acceptance of this proposal and closing date. Pending execution
               of the Agreement (or termination of this proposal), Shareholders
               and Companies agree that they will conduct its business only in
               the ordinary course and not engage in any extraordinary
               transaction without NESCO's prior consent. Companies agree that,
               prior to Closing of the Transaction, there will be no disposal of
               any asset with a value greater than $1,000.

                    14.  Closing Date - The closing date shall be on or before 
                         ------------       
               January 15, 1999. The Effective Date of the Transaction will be
               January 1, 1999.

                    15.  Term of Agreement - If not executed by Stockholders, 
                         -----------------            
               this Agreement shall expire at 5:00 PM CDST, Friday, December 18,
               1998.

                    If the foregoing is acceptable to you, please signify by
               signing this letter in the space provided below and returning to
               me a copy bearing original signatures.
               
               Sincerely,

               National Environmental Service Company
 
               _____________________________________

               Eddy Patterson - Chairman & CEO

               AGREED AND ACCEPTED:

               __________________________________
               Kenneth L. Hadnott - Shareholder

               __________________________________
               Charles D. Harrison - Shareholder

               _______________________________
               Brian Gray - Shareholder
<PAGE>
 
               NESCO Definitive Agreement - December 16, 1998 - Page 7

               THH Alpha, Inc.

               _________________________________
               ______________________________
               President


               THH Services, LTD.

               _________________________________
               ______________________________
               President


               UTTS Services, LTD.

               _________________________________
               ____________________________


               UTTS, Inc.

               _________________________________
               ____________________________
               President


               Carolina Drilling Services, Inc.

               ________________________________
               ____________________________
               President